An easy and effective way to make the perfect genoise sponge cake. Preheat the oven to 350 Fahrenheit, line 2 6-inch baking pans (or 1 9-inch) with parchment paper on the bottom only. Don't grease the sides. Set aside. Melt the butter in a sauce pan or microwave, set aside to cool slightly. Mix the flour and cornstarch together in a small bowl.
In contrast to other sponges, the genoise doesn’t use any chemical leavening agents. Instead, the volume is provided by suspending air in the cake batter – hence the need for vigorious whisking to encourage bubbles to form in the mixture, sometimes sped up by holding the bowl of eggs and sugar over a bain marie, or pan of just-simmering water.
Genoise cakes are Italian and French in their origin and named for the city of Genoa in Italy. You may be familiar with another sponge cake: Angel Food which is popular here in the U.S. Angel food cakes use only the egg whites to make a meringue but genoise uses whole eggs. What does a genoise sponge taste like?
Genoise will dry very quickly when left on its own; it's best to fill and finish it as soon as possible. Store any cake topped and/or filled with perishable ingredients in the refrigerator, well wrapped, for several days.