Every kitchen should have a sponge cake in the freezer for "dessert emergencies" since it freezes exceptionally well, even for weeks if wrapped properly, and can be used as the foundation of a quick and tasty dessert.
In making a sponge cake, the weight of the eggs is the basis for determining the quantity of the remaining ingredients. In the most common type of sponge cakes, the sugar and flour weights are 3 ounces (85 g) per 8 ounces (225 g) of eggs. If the recipe calls for butter, the amount is about half of the weight of the sugar and flour (1.5 ounces/ 42.5 g) and is added at the end.
The amount of sugar is vital to the end result. If too little sugar is used, the cake will become tough, as the batter will have too much flour in proportion to the other ingredients. The texture will become dense and the crust will become too dark. If the amount of egg yolks is increased in the cake, especially if an equal ratio of sugar, flour and eggs is used to create a heavy sponge, the yolks will reduce the amount of water content causing difficulties for the sugar to dissolve.
Bread flour has gluten to help bind and hold the cake's structure. Too much gluten, on the other hand, will make the batter rubbery and the result will be a tough and chewy sponge. If cake flour is used, the structure will collapse partially when baked even though the sponge will be lighter and tender. It is best to blend the two into a combination which will create the perfect sponge.
The volume and the light texture of a sponge cake comes from the air whipped into the eggs. When adding other ingredients, be careful not to break the air bubbles since they are the only leaven the cake relies on. Never stir the flour into the batter or use mixers; instead, fold the flour, eventual nuts and sweeteners (such as chocolate and butter) very carefully with a rubber spatula or your hand.
Butter will give flavor to the cake, extend the cake’s shelf life and give the cake finer pore structure. The butter should be melted but not hot and added last into the batter. If the butter is added before the flour, the butter will create small lumps of flour which will be impossible to break without losing the volume created when the eggs were whipped.
If nuts or candied fruits are added to the sponge cake, the cake has to be one of the heavier ones since the pieces will settle on the bottom in a lighter version. The same will happen if the flour and butter were whipped into the batter. The result will be a cake that looks great when taken out from the oven, but that has two separate layers: a top layer looking like a regular sponge and a rubbery bottom part holding all the butter and nuts. The almonds or hazelnuts should be of such fine consistency that they can be sifted with the flour. This will ensure that they don't sink as easily. Because nuts absorb moisture, the content of flour must be reduced accordingly.