To make easy butter icing, mix icing sugar and butter until desired amount and flavour are achieved. Add food colouring if desired.
Just a little addition. I've found when making these increasing the ingredients to match the eggs weight is useful so if two eggs weigh 120g use that much sugar, butter and flour which with a teaspoon of baking powder plus a good whisk of the mixture before adding flour makes them rise much better. - 01 Oct 2012