Chop the chocolate and melt it over a warm water bath, let it cool down lukewarm.
Cream butter and sugar for about 8 minutes. Stir in eggs one after the other. Mix flour, baking powder and cocoa and also alternately stir in with the sour cream. Mix in chocolate.
Pour the chocolate dough into a greased springform pan (26 cm Ø) and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 45 minutes. Take out and let cool down. Dust with icing sugar. Decorate the chocolate cake as you like with chocolate rolls.