Stir the butter until foamy, gradually stir in the sugar and eggs, stir in the flour mixed with baking powder and the hazelnuts spoon by spoon. Halve the dough, mix in half of the cocoa and rum.
Alternately pour the dough into a prepared tin (Gugelhupf or loaf tin) and bake in the preheated oven at 160 ° for about 40 minutes on the middle rail (test with chopsticks).