Cream together the butter and sugar until light and fluffy. Add the egg and vanilla and beat well. Fold in the flour and enough milk to make a dropping consistency.
Paper line a muffin baking tray. Spoon into 12 x 1/3 cup capacity muffin pans.
Bake in a moderate oven 180°C for 20 minutes or until cooked when tested.
Cool on a wire rack for 10 minutes before turning out of pan to cool completely. Store in an airtight container until required.
Beat the extra butter until pale and creamy. Add the vanilla, and then beat in the icing sugar and milk in 2 batches.
Colour the buttercream and chill until firm enough to spread or pipe. Decorate the cakes with the buttercream then decorate with the Cadbury Dairy Milk Marvellous Creations eggs.