Many recipes on the Internet, including this one taken from the UK BBC Food website. Not sure if it's any good though, but it will give After School an idea of what butter icing is:
140g/5oz butter, softened
280g/10oz icing sugar
1-2 tbsp milk
a few drops food colouring
Method
1. Beat the butter in a large bowl until soft. Add half the icing sugar and beat until smooth.
2. Then add the remaining icing sugar with one tablespoon of the milk.
3. Beat until creamy then add the food colouring and mix well.
4. Add extra icing sugar to thicken or a little extra milk to make more runny if necessary.
N.B. I'm sure I remember using lemon juice or liqueurs instead of milk in recipes before (?) Can't tell you if icing turns rock solid when it's been in the 'fridge because cakes tend to get consumed well before it has a chance to...