Chop 100 g chocolate. Melt half of the chopped chocolate over a warm water bath.
Cream butter, 175 g sugar, vanillin sugar and 1 pinch of salt with the whisks of the hand mixer. Stir in 1 egg. Mix flour and baking powder, knead in 2-3 portions with the dough hooks of the hand mixer.
Mix in the rest of the chopped chocolate. Remove 1/4 of the dough, form into coarse crumbles with your hands and chill. Knead the melted chocolate under the remaining dough. Put the dough into a greased, square springform pan (24 x 24 cm) and press it down.
Press the edge up while doing so. Chill the dough for about 30 minutes.
In the meantime, separate 2 eggs. Mix cream cheese, sour cream, egg yolks, starch and 75 g sugar. Beat the egg whites with the whisks of the hand mixer until stiff, slowly pouring in 50 g sugar.
Carefully fold the beaten egg white into the cream cheese mixture.
Pour 1/3 of the cream cheese mixture onto the pastry base. Spread the dough crumbles on top. Pour the rest of the cheese mixture over it. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) on the lower rack for 30-35 minutes.
Chop 30 g chocolate. After the baking time turn off the oven, spread the chopped chocolate on the cake and let it melt on the cake for 20-30 minutes with the oven door slightly open. Remove the cake and let it cool on a cake rack.