Peel oranges so that the white skin is completely removed. Bring mulled wine to the boil in a pot, remove from the heat and put oranges in. Cover with foil and leave to stand overnight.
Finely grind the walnuts in the universal chopper. Separate the eggs. Beat the egg whites, 1 packet of vanillin sugar and sugar with the whisks of the hand mixer until stiff. Stir in egg yolks. Carefully fold in starch, flour and walnuts.
Line the bottom of a springform pan (26 cm Ø) with baking paper. Fill in sponge cake mixture and smooth it down. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 12-15 minutes.
Take out, turn onto a cake rack, remove baking paper, let the base cool down.
Coarsely chop the chocolate coating and melt it over a warm water bath. Add 100 g cream and mix. Remove from the water bath, stir in orange liqueur and let it cool down while stirring occasionally. Whip 300 g cream with the whisk of the hand mixer until stiff, while pouring in 1 sachet of vanilla sugar.
Carefully stir the whipped cream into the lukewarm chocolate cream.
Remove the oranges from the mulled wine and drain well. Cut oranges into slices. Cut the sponge cake in half horizontally. Place the bottom cake layer on a cake plate, spread with half of the chocolate mousse and cover with half of the orange slices.
Place the second sponge cake on top and do the same with the rest of the mousse and oranges. Decorate with chocolate rolls and chill for at least 2 hours.