Melt the butter in a water bath. In the meantime, mix the flour and baking powder and beat the sugar, vanilla sugar and eggs until frothy. Then add the butter to the egg-sugar mixture, milk and the flour mixture. Mix everything well. Then distribute half of the batter on the muffin cases and stir in a good tablespoon of Nutella into the rest. Pour the remaining chocolate batter into the muffin cases and fold in once with a fork. Then bake everything in the preheated oven at 160 ° for about 20-25 minutes.
When the muffins have cooled, you can decorate them as you like.