Heat oven to 180C/160C fan/gas 4. Line a 12-hole bun tin with fairy cake cases. Beat the butter and sugar in a big bowl with a wooden spoon until it is really soft, then add the lemon zest. slowly stir in the eggs, then fold in the flour until it is all combined. Use two teaspoons to divide the mixture between the cake cases. Bake for 18-20 mins until the cakes are golden and springy when you touch them. Cool the cakes on a wire rack.
While you wait for the cakes to cool, make the butter cream icing. Beat the butter until it is really soft. Gradually beat in the icing sugar, lemon zest and juice until it is thick and yummy. Spread the butter cream on top of the cakes with the back of a spoon, then decorate with sprinkles or sweeties.