Preheat oven to Gas 4, 180C, 160C Fan oven. Line a 12 hole deep muffin tin with paper cases.
Melt the butter and Bournville chocolate in a bowl over simmering water. Remove from heat, whisk in eggs and milk to combine.
Sieve the flour, cocoa and baking powder into a bowl, stir in the sugar. Add the butter and chocolate mixture to the dry ingredients and stir gently to combine but do not over-mix otherwise this will toughen the muffins. Fill the cases half full and bake for 15-18 mins or until well risen and firm. Cool completely on a wire rack and turn off the oven.
While the muffins are cooking, take a piece of greaseproof paper and place on a baking sheet or tray. Place your buttons on the paper and place the tray in the turned-off oven for 1 min to melt the chocolate. Remove from the oven and using the back of a teaspoon, carefully shape your buttons into Moo decorations. Put the tray somewhere cool for the chocolate to set. Sieve the icing sugar and gradually add enough water until it is thick, treacle-like consistency.
Place the muffins back in the tin (easier to ice this way) and spoon enough icing over the top of the muffins so that it is almost level with the top of the case. Leave to set for 5 mins. Carefully place your Moo decorations on the muffins and then leave until the icing is completely set.
All information is provided by Cadbury