Grease a tart mould with a removable base (28 cm Ø). Knead 125 g butter in pieces, 175 g flour, icing sugar and 1-2 tbsp. ice-cold water first with the dough hooks of the hand mixer, then quickly with your hands to a smooth dough.
Roll out the dough on a little flour until round (approx. 32 cm Ø). Put the dough into the form. Press on the edge with your hands. Chill for about 30 minutes.
Pre-bake in the preheated oven on the lowest level (electric oven: 175 °C/circulating air: 150 °C/gas: level 2) for approx. 15 minutes.
Chop the chocolate and melt in a hot water bath. Let it cool down a little. Separate the eggs. Mix cream cheese, sour cream, lemon juice, sugar and vanilla sugar with the whisk of the hand mixer.
First stir in the egg yolk, then the flour. Stir in chocolate. Beat egg whites until stiff and fold in.
Spread the cheese mixture into the mould. Continue baking at the same temperature for about 1 hour. Let it cool down and dust with cocoa.