A custard is a thick, rich, creamy and pudding-like dessert made from gently cooking or baking and then refrigerating into a semi-rigid gel viscous sauce when cooled or refrigerated.

All custards are made basically the same ingredients: mainly eggs and/or yolks, as well as cream or milk, sugar and usually salt and flavorings. A small amount of starch such as flour, cornstarch, arrowroot and potato starch can be found in some custard recipes, while not in others. They range from being the entire dessert, such as a Cheesecake, Crème Brûlée or Flan, Pumpkin or Coconut custard pies, to being a part of a dessert such as Pastry Cream used when making éclairs and cream puffs. 

Custards as we know them today date back to the Middle Ages when it was used as a filling for a flan or a tart. The word custard is derived from "crustade" which is a tart with a crust. After the 16th century fruit creams became popular and it was about this time that custards were made in individual dishes rather than a filling in a crust.

DEFINING CUSTARD DESSERTS: Just as most baked products are essentially flour protein structures, custards are egg protein structures making them sensitive to heat and temperature.

The differences between custards are the result of how they're prepared, ingredients and mixing.

Custards are prepared in two ways: stirred or cooked on top of the stove or baked in the oven. 

    - Stirred or stovetop custards never get as thick as a baked custard have because their gel formation, primarily from eggs, is interrupted by stirring before it totally sets. They are often used as a sauce or as an ice cream base. For stirred custard mixtures, the eggs are cooked to the proper doneness when a thin film adheres to a metal spoon dipped into the custard. This point of coating a metal spoon is 20 to 30 degrees below boiling. Stirred custards should not boil. The finished product should be soft and thickened but not set. Stirred custards will thicken slightly after refrigeration.

    - Baked custards, the lightest of all, contain milk rather than cream and a relatively low ratio of eggs. As it is not unmolded, this custard does not need the extra thickening power of additional egg yolks. The gel is not stirred during the baking process, thus giving a more solid texture. Baked custard mixtures are done when a metal knife inserted off center comes out clean. The very center still may not be quite done, but the heat retained in the mixture will continue to cook it after removal from the oven. Cooking longer may result in a curdled and/or weeping custard. Cooking a shorter period may result in a thickened but not set custard.

Custard types: 

BAVARIAN CREAM: A cold dessert composed of a rich custard, whipped cream, various flavorings (fruit puree, chocolate, liqueurs and so on) and gelatin. The mixture may be spooned into stemmed glasses or into a decorative mold to be unmolded when set.

CHARLOTTE: This classic molded dessert begins with a mold lined with sponge cake, ladyfingers or buttered bread. The traditional charlotte container is pail-shaped, but almost any mold is acceptable. The lined mold is then filled with layers (or a mixture) of fruit and custard or whipped cream that has been fortified with gelatin. The dessert is chilled thoroughly and unmolded before serving. 

Charlotte Russe, said to have been created for the Russian Czar Alexander, is a ladyfinger shell filled with the ethereal bavarian cream, and decorated elaborately with whipped-cream rosettes. 

The classic Apple Charlotte is a buttered-bread shell filled with spiced, sautéed apples. Unlike other charlottes, this one is baked and served hot.

CHEESECAKES: A cheesecake is basically a custard baked in the oven. It's sensitive to being baked under direct heat or being cooled too fast. The end result is usually a crack down the middle. 

CRÈME ANGLAISE: The French term for a rich custard sauce that can be served hot or cold over cake, fruit or other dessert. (See also: Cream or Custard Sauce or Vanilla Sauce).  

CRÈME BRULEE: is French for burnt custard baked in the oven. This delicate, silken, and sinfully rich dessert, which blends the cool velvet of custard with a crisp, caramelized layer of sugar, from which it derives it name, that sits atop its creamy base.  

CRÈME CARAMEL: is a custard baked in a ramekin or mold with a layer of caramel at the bottom, often thought of as Flan's cousin. In some respects, it is similar to the crème Brulee that you see on many restaurant menus. They are both custards with a caramel layer. 

But crème caramel is made from egg yolks and egg whites (crème Brulee is made with yolks only) and its caramel is of a softer, more liquid consistency on the bottom (vs. the brittle caramel topping on a crème Brulee). When you unmold the crème caramel, there will be a topping of caramel on the custard as well as a sauce of caramel that pools around the custard. In Italy it's known as crema caramella and in Spain as flan.

CRÈME (Pastry Cream)

CUSTARD PIES: A baked custard pie is when the filling, an uncooked custard, is poured into a pie shell and both are baked together.  (Versus a cream pie, is where the filling is made from a cooked custard or mousse, spread in a cooled, pre-baked pie shell.) A Pumpkin Pie is considered to be a custard pie as is a Pecan Pie.

FLAN: Flan is commonly used as a term to describe the Spanish or Mexican version of Crème caramel. When inverted, the caramel baked on the bottom becomes the topping.

ICE CREAM: French ice cream has a cooked egg-custard base. 

MOUSSE: A French term meaning "froth" or "foam," mousse is a rich, airy dish that can be either sweet or savory and hot or cold. Cold dessert mousses are usually made with fruit puree or a flavoring such as chocolate.

A mousse is similar to a Bavarian crème, which is a custard or a chiffon, in that it is light and airy due to the addition of whipped egg whites, whipped cream or both. They are usually fortified with gelatin (not as much in chocolate mousse recipes) because they are generally softer than the Bavarian or chiffon. Mousses tend to be too soft to mold into a semirigid form, ie a gelatin dessert, so they are usually piped into the final dessert presentation and garnished with additional whipped cream just prior to serving.

Savory mousses can be made from meat, fish, shellfish, foie gras, cheese or even vegetables. Hot mousses usually get their light texture from the addition of beaten egg whites. They're generally baked in a water bath to prevent the mixture from curdling.

PASTRY CREAM: Considered the mother of all creams!

POT DE CRÈME: French for "pot of cream," is traditionally served in small, lidded porcelain cups and are rich custardy desserts, relatives of creme brulee, creme caramel, and flan. 

PUDDING: Similar to a custard.

QUICHE

VANILLA SAUCE: Vanilla sauce is considered a "mother sauce" in pastry. It is used as the base in many creams, mousses, ice cream and desserts. It is sometimes referred to as - Crème Anglaise, Cream or Custard Sauce and English crème/sauce. It is based on milk, sugar and egg yolks.

ZABAGLIONE: is a classic Italian dessert cooked on the stovetop, that is an egg custard flavored with sweet wine. While traditional Tiramisu calls for raw egg yolks, often recipes use Zabaglione. Of course, you also serve just it on its own over a slice of plain cake or ladyfingers or even with fresh berries. 

EGGS, A PRIME INGREDIENT IN CUSTARDS: 

A large portion of the discussion of stirred and baked custard hinges on eggs, such as whole eggs, whole eggs plus yolks and just yolks.

Eggs are the main thickener in most custards and the yolks make them smooth and rich. Both egg yolks and whites contain proteins, which change from liquid to solid, called coagulation, when cooked or baked. This means that the liquid egg becomes firmer. As heating continues the egg eventually becomes semi-gelled or fully gelled when cooled, giving you the custard's texture. The less eggs in a custard recipe, the cooking time increases and so does the coagulation time.

Because eggs are the primary structural ingredient of a custard, it's important to use fresh, large Grade A ones -- a negative difference in the custard can be seen when using frozen, old, dried or egg substitutes. 

Eggs are sensitive to high heat and thus, all custard recipes require slow cooking or baking and gentle heat. If a recipe contains starch (cornstarch or flour) it HELPS to prevent the eggs from overcooking and gives more lead way between success and failure of the egg-rich mixture, but problems can still occur. 

Whether a recipe contains starch, determines how the custard is cooked or baked:

No matter how a custard is cooked or baked, it is important to remove the custard the minute it's done from the stove or oven AND the double boiler or waterbath; the ingredients continue to cook if you don't.

All custards break down over time, so it's important to consume them right away. And, remember to keep all custard desserts and fillings refrigerated!

How Eggs Thicken: Both egg yolks and whites contain protein. They are shaped like coils or springs or complex wads that are all separate from one another. You can see through an egg white because the proteins are not attached! 

When eggs are heated, their proteins unwind (called denaturing) and break apart from their tightly bound bundles, bump up against one another, and adhere to form loose, flat and long strands. These strands are linked together in a three-dimensional mesh. You can see the result of this process with egg whites because they turn from clear to opaque, forming a solid gel. Liquid gets trapped in these strands, and this causes the mixture to thicken. 

Gentle cooking, such as baking a custard without starch in a waterbath, keeps the egg proteins loose and soft until the mixture gets hot enough, at around 160 degrees F. The constant stirring with a stove top recipe maintains an even temperature so the protein bonds don't form too early, which are allowed to set undisturbed in baked versions. 

When starch is present in a custard recipe, it HELPS to prevent the unwound egg proteins from joining together too soon, thus when heated to too high a temperature and then cooled, the custard won't scramble or crack. 

But despite the gentle heat, if a custard is heated above 180 degrees F, the more tightly the proteins join together. They becoming thicker, curdling and squeezing out all the water which you see evidence of coming from little tunnels in the custard, called synersis. The egg proteins will overcoagulate from too much heat which eventually shrink when cooled. 

You can also see overcooking or exposure to too much heat in an cracked cheesecake. Overheated eggs shrink when cooled causing with a large crack running through its center or tiny cracks all over its top. 

When a starch-based custard mixture gets hot enough and boils at around 212 degrees F, the egg proteins simply join together. However, if you undercook such a custard they won't join together and gel. Yolks have a starch digesting enzyme called alpha-amylase. In order for a successful gelling of a starch in the recipe, the enzyme has to be killed by cooking the custard almost to boiling (a little under 212 degrees F). Otherwise the left-over enzymes digest all of the nice firm starch gel and your custard is nothing but liquid.

OTHER CUSTARD INGREDIENTS AND TIPS:

Besides eggs, custards contain either milk or cream or both. Cream contributes to the thickness of the custard, as those made with water or skim milk will not gel or thicken. Many recipes direct you to scald them before using; this is a holdover from the days of unpasteurized milk. Scalding does, however, shorten cooking time because the milk is already hot; it also ensures that the sugar dissolves completely in the custard base before baking, so I recommend this step.

Custards contribute to a plain back drop and can be easily paired with various flavorings to create exciting and exotic variations on the basic theme. Some flavorings can include melted chocolate, vanilla extract (always use pure extract, not imitation) or a medley of aromatic spices, fruits, and citrus flavors. Before baking, grate a little nutmeg over the top. Remember to add other flavorings AFTER the custard has cooked and is still hot and not firmly set (except for vanilla bean seeds which are added to infuse flavor into the cream). You don't want to loose flavor when the extracts are cooked with the rest of the ingredients and you don't want to add when the custard has cooled. Acids, such as freshly squeezed lemon or orange juice decreases coagulation time and temperature so, always add after the custard has finished cooking. Any stirring when the custard has cooled to incorporate the flavorings will thin it. 

With sauces, such as custard, containing dairy and milk products, form skins on top after cooling. It is caused by the casein in the dairy product. Some cooks dot the top of the custard with butter (open one end of a butter stick and dot it on the surface of the custard while still hot) to form a thin fat glaze. Others press plastic wrap on the surface when the custard is still hot or warm, so when it cools, a skin won't form.

Sugar is also important to a custard as the addition of it in a recipe results in a softer custard. Sugar also increases the coagulation temperature and time. Don't dump the sugar directly onto the eggs and let it sit; this causes the yolks to "burn" into hard little lumps that detract from your creamy custard. Rather, add the sugar while your whisk is moving; this way, the sugar will be gradually incorporated into the eggs. However, too much sugar prevents the eggs from coagulating.

Some information from Shirley Corriher, CookWise