Melt the couverture over a warm water bath. In the meantime, whisk the fat, sugar, salt and vanilla sugar with the whisk of the hand mixer. Add curd and sour cream and stir in.
Stir in the eggs one by one. Stir in pudding powder and semolina. Halve the dough. Stir the melted chocolate coating into one half. Grease the box form (25 cm long; 1.5 litres capacity) well.
Cut the baking parchment a little wider than the tin and about 40 cm long. Knock out the tin from the long side and let it protrude on both sides. Press on the baking parchment well. Spread breadcrumbs on the narrow sides.
Pour in dark dough. Pour light-coloured dough on top and marble with a fork. Bake in the preheated oven (electric cooker: 200°C/ convection oven: 175 °C/ gas: level 3) for approx. 60 minutes. Cover if necessary 15 minutes before the end of the baking time.
Take the cake out of the oven and let it cool down on a rack. Loosen the narrow sides with a knife. Lift the cake out of the tin with Hiolfe baking paper. Dust the cake with cocoa.