Line a 12-section bun tray with paper or foil cake cases. Put all the cake ingredients in a bowl and beat with a hand-held electric whisk for about a minute until light and creamy. Divide the cake mixture between the paper or foil cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15–18 minutes or until well risen and just firm to the touch. Leave to cool in the tin.
Mix together the icing sugar, vanilla extract and enough water in a bowl to make a smooth icing. Divide the icing between 3 bowls and colour each batch with a different food colouring.
Remove the cakes from the tin, cover the tops with the different icings and decorate with jelly beans. Leave for 30 minutes for the icing to set.