Beat the eggs with the sugar until frothy. Add the butter and keep stirring. Add vanilla sugar. Sieve the flour with the baking powder and gradually stir into the foam mixture. Gradually add milk until the batter is loose but not too runny. Then add the cocoa powder and stir in well. Cut the milk chocolate into small pieces (about 5 mm cubes) and carefully fold them into the batter.
Fill the muffin molds about 2/3 with batter, press in a giant chocolate toffee and cover the toffees with a little batter. Bake for 25-30 minutes.
After the muffins have cooled down, melt the dark chocolate in a water bath and brush the muffins with it carefully, but not too sparingly.