Preheat oven to 180C (160c for fan oven) / Gas Mark 4. Place the cupcake cases onto a bun tin.
Put the butter and sugar into a bowl. Mix with a wooden spoon or a mixer until light and fluffy. Break the eggs, add to the bowl and stir. Add the vanilla extract, flour and baking powder. Mix together (if its really thick, add 1 or 2 tablespoons of milk or water).
Divide the mixture into cupcake cases (makes 12-18 depending on the size of the cases). Bake for about 15-25 minutes, until golden brown and firm to the touch. Leave to cool before decorating.
To make the icing, beat the butter or margarine until soft then gradually beat in the icing sugar until light and fluffy. Spread or pipe on top of the cakes.