Chop the couverture and melt in a hot water bath or in the microwave. Beat eggs, powdered sugar, salt and vanilla sugar with a food processor for 10 minutes until creamy.
Slowly pour in the oil and egg liqueur while stirring, stir well. Sift flour, starch and baking powder mixed over the mixture. Stir in briefly. Divide the dough. Mix one half well with the couverture, nuts and cocoa.
Alternately pour or spoon the two fairly liquid doughs into a greased Guglhupf tin (2 l contents) that has been dusted with flour. Bake in the preheated oven at 175 degrees (convection 150 degrees) on the middle rack for 1 hour 10 minutes.
Take the cake tin out of the oven and let it rest for 5 minutes. Overturn the cake on a wire rack. Let cool down lukewarm. With 1 tablespoon. Dust the powdered sugar.