For the dough, stir the eggs, water, powdered sugar and vanilla sugar until frothy. Gradually stir in the oil. Mix flour with baking powder and cocoa, stir in quickly and distribute the dough on 2 greased baking trays (36cm x 32cm). Bake in the preheated oven for about 15-20 minutes at 175 °. Let cool down.
Mix the sour cream or creme fraiche with the yoghurt, sugar and vanilla sugar.
Do not whip the cream very hard. Squeeze out the soaked gelatine, melt over low heat, mix with a little sour cream and yoghurt mixture and then add to the remaining mixture.
Carefully fold in the whipped cream.
Spread the sour cream and cream mixture on the cooled dough and smooth it out (it is best to use a baking frame) and place the second dough on top.
For the chocolate icing, heat the cream in a small saucepan. Remove the saucepan from the heat, add the chocolate and butter and stir until the chocolate has melted. Slowly pour the icing onto the cake and distribute it evenly.
Garnish with white chocolate.
Put in a cold place, best prepared 1 day in advance.