So, here's how I made them:
-100g of soft butter
-120g of sugar
-120g of flour
-1 tablespoon of cocoa powder
-1/2 teaspoon of baking powder
-100g of Nutella
1) Preheat the oven to 170 degrees C.
2) Mix the butter and sugar in a bowl, add the eggs.
3) Mix flour, cocoa powder and baking powder in a different bowl and add to the first mixture.
4) Then add Nutella and stir or mix again for some seconds.
5) Put the dough in the muffin cases and bake for 20 minutes.
6) Let them cool down and you are done!
Of course you can add a frosting but I preferred them this way.
I really like them and they are simple to prepare.
Let me know if you try them out one day!