For the chocolate cupcakes, stir all the batter ingredients together. Pour the batter into muffin tins and bake in a preheated oven at 180° for about 20 minutes.
For the topping, melt the chocolate in a water bath, stir in the sugar and carefully fold in the sour cream. Fill into a piping bag and pipe the topping onto the cooled muffins. Chill the chocolate cupcakes.