Cream the fat with the whisk of the hand mixer. Add salt, sugar and vanilla sugar. Stir in the eggs one after the other. Mix flour and baking powder, sieve onto the fat cream.
Stir in together with crème fraiche. Halve the dough. Stir in half of the cocoa and milk. Put light and dark dough in layers into a greased, flour-sprinkled box form (25 cm long, 1.5 litres capacity).
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 1 hour. Leave to rest in the pan for 10 minutes. Turn out onto a cake rack and let it cool down. Dust with icing sugar.
Whipped cream is delicious with it.