Mix the eggs well with the soft butter. Mix the flour, sugar, vanilla sugar and baking powder, add to the butter-egg mixture and stir. Now divide this dough into two roughly equal parts.
Then chop the dark chocolate with the knife into small to somewhat coarser pieces and add to part of the dough together with the Nutella, milk and cocoa and mix. Pour the eggnog in the other part of the batter.
Now first pour the light batter into the muffin pan and then pour the dark batter over it so that the molds are about 2/3 full.
Then bake them in a preheated oven at 180 ° C for around 15-18 minutes. It is better to take out the muffins earlier, otherwise they will get too dry.