Roughly chop the chocolate and melt it with butter over a warm water bath. Add eggs, flour and sugar and mix with the mixer. Pour the batter into muffin molds and press a piece of chocolate into the batter. Bake in a preheated oven (fan oven 150-160 ° C) for 15 minutes.
The amounts given are sufficient for approx. Four muffins, for a whole muffin tray simply triple the amount of ingredients.