Preheat the oven to 200ºC/400ºF/Gas 6. Line a 12 hole muffin tin with paper muffin cases.
To make the muffins, sift the flour, sugar, cocoa and baking powder into a large bowl. Mix the butter, milk, eggs and vanilla together and stir into the flour mixture. Beat until the mixture is just smooth – do not over beat.
Spoon the mixture into the paper cases and bake for 20-25 minutes or until risen and firm to the touch. Cool on a wire rack.
To decorate, beat the icing sugar and butter together. Blend the cocoa and water to a smooth paste and beat into the butter mix. Spread over the top of the muffins and decorate with Easter eggs.