Melt butter in a microwave
Place melted butter in a bowl
Add sugar and whisk well.
Add egg, whisk again
Add the milk, whisk to have a homogeneous mixture
Add the sifted flour with baking powder and a pinch of salt, give a few lashes without working the dough too much.
Place the dough in a pastry bag, and fill 6 medium muffin cups halfway.
Add the teaspoon of Nutella, and fill with the remaining dough
With a wooden skewer, make movements in the dough to give a marbled effect to the muffins
Bake in an oven preheated to 200°C for 5 minutes, then lower the temperature down to 175°C and bake between 15 and 20 minutes, until you have a nice brown colour in the muffins.