1. Line the muffin tin with the paper cases. Put all the cake ingredients into a bowl, and beat with an electric whisk until evenly combined and smooth.
2. Divide the mixture among the paper cases. Bake in a preheated oven at 180°C (160°C fan, Gas 4) for 20–25 minutes until well risen and light golden brown. Transfer the cakes in their cases to a wire rack, and leave to cool.
3. Make the icing: put the butter and half of the icing sugar into a bowl, and beat with an electric whisk until evenly combined and smooth.
