With considerable beating, room temperature egg yolks can hold air bubbles in their fat much the same way that butter does. (With a Stand Mixer, use the whip attachment).

Egg yolks contain valuable emulsifiers or fats which moisturize a baked good (versus egg whites which have a drying effect). They are thicker than beaten whole eggs, and can stand for 5 - 8 minutes before starting to deflate.

Sugar is often added to the yolks. It stabilizes egg yolks much the same way it does with beaten egg whites.  Superfine sugar is recommended because it dissolves easily and makes a better cake or one with a very fine texture. 

When working with sugar and yolks, break up the yolks first with a wire whisk or fork. Then, add the sugar and mix them together quickly and evenly. Use immediately in the recipe.

TO DO:

Separate yolks from whites, and place them in separate bowls. Cold eggs separate more easily than room temperature ones, but the separated egg yolks need to be warmed to room temperature when using. 

With an electric mixer on medium to medium-high, beat the yolks for 3 to 5 minutes. Afterwards, if sugar is added, do so in a steady stream while beating. 

The yolks are sufficiently beaten when thick and pale in color and form ribbons when the beaters are raised.