Crush the butter and chocolate and melt in a bowl over the boiling water bath. Then let the mixture cool down and stir in the beer.
Mix the flour, cocoa and baking powder in a bowl. In another bowl, beat the eggs and sugar for 3 minutes until frothy. Add the chocolate mixture and eggs to the flour and stir everything well. Fill into a muffin tin with 12 cavities and bake at 180 ° C / speed 4 in the gas oven for 20-25 minutes.
Meanwhile, mix the sour cream and powdered sugar to a glaze. Let the muffins cool and brush with the icing.