For the banana slices, beat the eggs with sugar and vanilla sugar until foamy. Mix flour with cocoa and baking powder and fold in loosely, oil as well.
Spread the mixture on a baking tray lined with baking paper. Bake in the preheated oven at 180 degrees for 20-25 minutes. When cooled, spread with jam.
Cut bananas into slices and spread on the cake. Soak gelatine and dissolve in a little heated whipped cream.
Whip remaining whipped cream until stiff, fold in yogurt, gelatine and chocolate flakes, spread on cake, chill until banana slices are firm.