Preheat oven to 180 °C convection oven.
In a bowl, whisk together flour, baking powder and salt. In another bowl, beat the eggs with sugar and vanilla sugar until foamy. Add the melted butter, keep mixing, then stir in the milk and yogurt. Now add the flour mixture and keep mixing well. My tip if there are lumps in the batter: the blender! Grease the muffin tin (butter and flour) or make paper cups. Fill the muffin cups 1/3 full with batter.
Form a small hollow in the middle. Put a teaspoon of Nutella in it. Cover with batter. Bake the muffins in the oven for about 20 minutes.
Take the muffins out of the oven and remove them after 5 minutes.
