For the Feinspitz chocolate sponge cake for tarts and roulades, preheat the oven to 160° convection oven. Cover a springform pan (26 cm) with baking paper.
Separate the eggs and beat the yolks with the sugar, vanilla sugar and hot water for 10-15 minutes until foamy. Mix the flour well with the cornstarch, cocoa and baking powder, sift over the yolk mixture and fold in. Beat the egg whites with a pinch of salt and the baking sugar and fold into the mixture.
Pour into the springform pan and bake on the middle shelf for about 35-40 minutes. If the surface darkens too quickly, cover with aluminum foil.
The recipe is good for a base that you can cut twice. For a roulade, spread the mixture on a baking sheet lined with parchment paper and bake for about 15-20 minutes.