In a bowl, mix the flour, cocoa powder, butter, sugar, baking powder and salt. While stirring, stir in the milk, then the egg and Nutella until a smooth mixture is formed.
Line a muffin tin with paper muffin liners, if there aren`t any, grease the hollows. Fill the wells 2/3 with the mixture and bake for 15-20 minutes.
First let the muffins cool down completely, then cut a small circle out of the top of the muffins, pour some Nutella into the resulting cavity and use the cut-out piece of dough again to seal the muffins.
Whisk all ingredients for the glaze until a fluffy, smooth mass is formed. Spread over the filled cupcakes with a knife.