Mix the butter, egg, sugar and cocoa powder until frothy and gradually sift in the baking powder and flour. Use the hand mixer to make loose sprinkles. Then halve the amount and press one half into a greased springform pan (24 cm).
Filling:
Mix the butter, sugar and egg yolks together until frothy. Stir in the custard powder, vanilla sugar, lemon juice and quark. Finally fold in the stiff egg white.
Place the filling on the dough base and top with the remaining crumble. Bake at 175 degrees for about 50 minutes. Let cool down before cutting.