Separate eggs. Beat the egg whites until stiff. Gradually pour in the sugar. Stir to a glossy mixture. Stir in the egg yolks.
Mix the flour with cornstarch, cocoa powder (for the dark dough) or vanilla pudding powder (for the light dough) and baking powder and sift over it. Carefully fold in everything.
Put the sponge cake mixture in a springform pan (26 cm) lined with baking paper and smooth it out.
Bake in the preheated oven (electric stove: 200 degrees, convection: 175 degrees, gas stove: level 3) for about 30 minutes.