Mix butter, sugar, eggs, flour, baking soda and cocoa powder in a bowl to form a smooth dough. Fold in the sour cream with a spoon. Now fill the batter into the muffin cups. Bake at 180 degrees for 20 minutes.
For the topping, melt the chocolate in a water bath. Let cool down briefly. Now stir in the sugar and sour cream. Spread the topping on the cupcakes, decorate with chocolate chips. Let it set in the refrigerator and then serve.