Cream butter with sugar until fluffy. Then stir in the egg and next the milk and chocolate sauce. Finally, mix in the flour with the baking powder until you have a smooth mixture.
Fill the muffin tins two-thirds full at a time with the mixture. Bake for 20 minutes at 180 °C (top and bottom heat).
Meanwhile, beat the butter for the frosting until fluffy and stir in the sugar and cocoa. If the frosting doesn’t taste like chocolate or isn’t sweet enough, simply stir in more cocoa or sugar to taste.
When the muffins have cooled, spread with the frosting (or pipe it on with a piping bag) and refrigerate.