Preheat oven to gas 4, 180˚C, fan 160˚C. Line a 12-hole bun tin with cupcake cases. Put the butter and sugar in a large bowl and beat with an electric beater until the mixture is light and fluffy. Beat in the eggs one by one and then beat in the flour and vanilla extract, plus a pinch of salt. The mixture should fall easily off your spoon; add a drop of milk if it is too stiff.
Put the mixture into the cupcake cases. Bake for 15-20 minutes, until the cakes have risen and are firm. As soon as you can, lift out of their tins and cool on a wire rack.
For the icing, beat the butter until it's soft, then beat in the icing sugar until you have a soft buttercream. Divide the mix into 3 portions and colour each one pale pink, green or yellow. Ice the cakes with the different colours, either using a palette knife or piping it on. Decorate with the pearl swirls or silver balls, or decorations of your choice.
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