Preheat the oven to 210 ° C. Brush the muffin tin with oil or melted butter. Optionally, paper baking cases can also be placed in the hollows.
Put the flour, baking powder and cocoa powder in a bowl. Stir in sugar and chopped dark chocolate and make a well in the middle.
Whisk eggs with buttermilk in a bowl, pour the melted butter into the hollow and stir with a spoon until a batter is formed. Do not stir for too long, the dough should still have lumps.
Fill each cavity of the muffin pan up to about three quarters with batter, sprinkle a total of 2 tablespoons of chopped dark chocolate or chocolate chips over it, press lightly and bake for 30 minutes, until a stick that is pierced in the middle of a muffin stays clean when pulled out. Let the muffins cool down in the tin for 5 minutes, then remove them and let them cool completely on a wire rack.