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Heat oven to 190C/fan 170C/gas 5. Line a 12-cup muffin tin with brown muffin cases. Beat the first 5 ingredients to a smooth batter and spoon between the cases, almost filling them to the top. You may have a little left over. Bake for 20-25 mins until risen and spongy. Cool on a rack.
Beat the butter and icing sugar until smooth. Slice off the very tops of the cakes and cut each piece in half. Spread a generous layer of icing over each cake.
Working on one cake at a time, squirt a pea-sized blob of orange icing onto two Maltesers and use to fix a brown M&M on each. Sit the eyes, two pieces of cake top (curved edge up) and a jelly diamond on the icing to make an owl.