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Heat oven to 180C/160C fan/gas 4. Line a 12-bun tin with fairy cake cases. Put the sugar, butter, flour, eggs, vanilla and banana in a big bowl together. Beat with an electric whisk until banana is mashed and everything mixed. Divide between the cases and bake for 20-22 mins until a skewer poked in comes out clean. Cool.
To decorate, mix the icing sugar with enough water to be spoonable, but not too runny or it will dribble off the cakes. Spread some over each cake. Knead some food colouring into the regal icing to make a bright red colour. Roll 12 different-size balls, dip one side or the top of each in some sprinkles and stick one on each cake.