For the sponge dough, beat the egg, sugar and water until foamy. Lightly fold in the flour (for a larger mass, of about 3 eggs, it is recommended to mix 1 pinch of baking powder into the flour).
Bake quickly in the preheated oven at 220 °C for about 5-6 minutes (bake the cake for about 10 minutes at 200 °C and about 40 minutes at 160 °C).
For the yogurt cream, mix yogurt well with sugar, vanilla sugar, lemon juice and rum.
Dissolve the gelatine. Stir one or two spoonfuls of the yogurt cream into the gelatine, only then stir in the complete mixture.
Finally, fold in the whipped cream.
Spread the cake base with jam, pour the cream over it, garnish with fruit of your choice and refrigerate for at least 6 hours.