Melt the butter and let it cool down a little. Roughly chop the chocolate. Mix flour, sugar, cocoa, espresso, baking powder and vanillin sugar in a bowl. Place the eggs in a mixing bowl and whisk until creamy. Allow butter to run in and stir in.
Mix in the flour mixture well with a mixing spoon. Stir in sour cream, then fold in chocolate. Pour the dough into a greased muffin tray (12 troughs) sprinkled with flour. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 25-30 minutes. Remove the muffins, let them cool down and dust with icing sugar
Waiting time approx. 1 hour