Melt the butter and let it cool down a little. Mix flour, 100 g sugar, 1 packet of vanillin sugar, baking powder and salt. Add the egg and butter and work into crumbles with the dough hooks of the hand mixer. Pour the dough into a greased square springform pan (24 x 24 cm) and press to a flat base. Press up an approx. 1 cm high rim.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 15 minutes. Let it cool down. Mix the mascarpone, quark, 75 g sugar, 1 packet of vanilla sugar, cream firming agent and half of the lemon juice with the whisks of the hand mixer to a smooth cream. Spread the cream on the base. Peel the bananas, halve each lengthwise and sprinkle with the remaining lemon juice. Spread the bananas on the cream. Chill for about 1 hour. Chop the chocolate coating and melt in a warm water bath. Use a tablespoon to spread the couverture in strips on the cake. Let it dry.
Chill for about 1 hour. Chop the chocolate coating and melt in a warm water bath. Use a tablespoon to spread the couverture in strips on the cake. Let it dry. Peel off the white chocolate with a peeler. Sprinkle the cake with it. Cut into pieces and decorate with balm