300g butter (preferably unsalted), at room temperature
600g icing sugar, sifted
2―3 tablespoons milk
Flavouring or colouring of your choice
Beat the butter until smooth, then add the sifted icing sugar. If necessary, add a couple of tablespoons of milk to achieve the right consistency (not too stiff, not too sloppy). Add flavouring such as vanilla or an essence such as orange, lemon, etc., then add the food colouring (see tips below) one drop at a time and mix well to blend the colour.
For the average novelty cake, I use roughly 500g butter to 1kg icing sugar, but obviously this depends on the cake in question. I always make a generous amount of buttercream icing so that I can use any surplus to ice other smaller cakes I might have at hand.