1. Position the oven rack in the lower third of the oven. Preheat the oven to 450 degrees.

2. In a small bowl, whisk together the cake flour and cornstarch.

3. Separate 2 of the eggs, placing the yolks in 1 large mixing bowl and the whites in another. To the yolks, add the additional yolk, the 2 remaining eggs, and ½ cup sugar. Beat on high speed 5 minutes or until thick, fluffy, and triple in volume. Beat in the vanilla.

4. Sift ½ the flour mixture over the egg mixture and fold it in gently but rapidly with a large balloon whisk, slotted skimmer, or rubber spatula until the flour has disappeared. Repeat with the remaining flour mixture.

5. Beat the egg whites until foamy, add the cream of tartar, and beat until soft peaks form when the beater is raised. Beat in the remaining 1 tablespoon sugar and beat until stiff peaks form when the beater is raised slowly. Fold the whites into the batter and pour into the prepared pan, using an angled metal spatula to level it.

6. Bake for 7 minutes or until golden brown, a cake tester comes out clean, and the cake is springy to the touch.

7. Loosen the edges with a small metal spatula or sharp knife and, lifting by a long edge of the liner or parchment overhang, gently slide the cake from the pan onto a flat surface. To use the biscuit for a round cake base or cutouts, allow to cool flat, covered with a clean dish towel. To use it for a roll, roll it up while still hot. If using a liner, tightly roll up the biscuit with the liner. (This keeps the biscuit especially moist.) If using parchment, flip the biscuit onto a clean dish towel, carefully remove the parchment, and roll it up tightly, towel and all. Cool on a rack. When ready to fill, unroll the biscuit. (If a liner was used, first detach the cake from liner and then replace it on the liner.) If using the syrup, sprinkle it on the cake before spreading it with 2 cups filling.

TO MAKE SYRUP:
In a small saucepan with a tight-fitting lid bring the sugar and water to a rolling boil, stirring constantly. Cover immediately, remove from the heat, and cool. Transfer to a liquid measuring cup and stir in the liqueur. If the syrup has evaporated slightly, add enough water to equal 1/3 cup syrup.

VARIATIONS:

Ginger Biscuit: 1 tablespoon ginger juice added to the yolk mixture cuts the sweetness and adds a unique, subtle flavor. (To make ginger juice, grate fresh ginger on a fine grater and press with your fingers to squeeze out as much juice as possible.)

Chocolate Biscuit

Almond Biscuit: My favorite of all biscuits--moist, tender, and flavorful because there are more almonds than flour. In place of the flour/cornstarch mixture, use 1/3 cup blanched, toasted, and finely ground almonds and 3 tablespoons unsifted cake flour.

Green Tea Biscuit: The moss-green tea of the Japanese tea ceremony adds a lovely color and exquisitely haunting flavor to the delicacy of biscuit. It must be served the day it is baked or the elusive flavor is lost. To make this biscuit, replace 1 tablespoon of the cornstarch with equal measure or weight of powdered green tea.