Pre-heat oven to 180ºC/350ºF/Gas Mark 4. Place the Baking Cases into a muffin tin.
Cream together the butter and sugar until light and fluffy. Gradually beat in the eggs, if the mixture starts to curdle, add a little flour.
Fold in the remaining flour and the cocoa powder with a metal spoon. Place spoonfuls of the mixture into the baking cases and bake for 15 - 20 minutes until well risen and firm to the touch. Remove from oven, leave to cool on a cooling rack.
Take a sharp knife and cut a circle out of the top of each cake at an angle about 1cm (½ inch) from the edge. Cut each circle in half and set aside.
Spoon the Chocolate Buttercream onto the hole in the cake.
Place the two halves of cake on top to resemble butterfly wings. Dust the top of each cake lightly with the sieved icing sugar.