Grease a square springform pan (24 x 24 cm; alternatively round, 26 cm Ø). For the cookie dough, chop 100 g chocolate. Melt half in a hot water bath. Whisk 200 g butter, 175 g sugar, vanilla sugar and 1 pinch of salt until creamy.
Stir in 1 egg. Mix 300 g flour and baking powder. Stir into the butter-sugar mixture. Stir in chopped chocolate. Approx. 1⁄4 Remove the dough and shape it roughly into pieces. Cover and put in a cool place.
Stir the melted chocolate into the remaining dough. With floured hands press into the mould to form a base, forming a rim. Chill for about 30 minutes.
In the meantime, separate 2 eggs for the cheese mixture. Beat the egg whites until stiff, while pouring in 50 g sugar. Stir cream cheese, sour cream, egg yolk, starch and 75 g sugar until smooth. Fold in the beaten egg white.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
Pour half the cheese mixture onto the cooled dough base. Spread cookie dough pieces on top. Add the rest of the cheese mixture. Bake in the hot oven on the bottom shelf for 30-35 minutes.
Chop 30 g chocolate. Turn off the oven, sprinkle the chopped chocolate on the cake. Let it cool down for 20-30 minutes with the oven door slightly open. Take out and let it cool down.