Chop the chocolate and melt over a warm water bath. Cream butter, 250 g sugar and 1 packet of vanilla sugar with the whisk of the hand mixer. Stir in 4 eggs one after the other. Mix flour and baking powder and stir in alternately with 75 ml milk. Stir in chocolate and cocoa.
Beat 2 eggs and 100 g sugar and 1 packet of vanilla sugar until creamy with the whisks of the hand mixer. Stir in curd and starch.
Pour 2/3 of the chocolate dough into a greased, large Savarovy tin (26 cm Ø; 2.75 litres capacity), dust with flour. Spread the dough a little bit high at the edge, pour the cheese mixture into the depression. Mix the rest of the dough with 50 ml milk and carefully spread it on the cheese mixture. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 1 hour, remove from oven and let cool on a cake rack. Turn out onto a cake plate while still lukewarm and let it cool down. Dust with icing sugar.