Put all the cupcake ingredients in a mixing bowl or a food processor and beat until smooth. Spoon the mixture into foil cake cases arranged in a greased 12-hole deep muffin tin. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15–18 minutes until well risen and the cakes spring back when gently pressed with a fingertip. Leave to cool in the tin.
Make the topping. Mix together the icing sugar, vanilla and enough of the water to make a smooth icing. Divide the icing among 3 bowls and colour each batch differently. Turn the cakes out of the tin, ice them and decorate with jelly beans. Leave for 30 minutes for the icing to set.