Melt butter in a small pot. Put the biscuits in a freezer bag and crumble them finely with a rolling pin. Mix them in a bowl with liquid butter. Line the bottom of a springform pan (26 cm Ø) with baking paper, press the biscuit-butter mixture firmly as a base and refrigerate.
Cut the vanilla pod lengthwise and scrape out the pulp. Mix quark, cream cheese, 150 g cream, lemon juice, starch, sugar, egg and vanilla pulp. Put the cheese mixture on the biscuit base and smooth it down. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 45 minutes. Remove from the oven and let it cool down on a cake rack. Carefully remove from the edge of the mould.
Melt the dark chocolate over a hot water bath and let it cool off lukewarm. Whip 250 g cream until stiff, carefully fold in chocolate. Smooth on the cake and refrigerate for about 2 hours. Lift the cake out of the mould, roughly slice the white chocolate and sprinkle the cake with it. Cut into pieces and serve.