This was taken from a recipe by K. Blackburn on Allrecipes.com: http://allrecipes.com/Recipe/chocolate-butter-creme-frosting/detail.aspx It is really delicious.
180g icing sugar
50g butter, softened
pinch of salt
1 tablespoon milk, add less to make stiffer for piping or add more to make softer for spreading
1 teaspoon vanilla extract
(Add 50 g chocolate, melted or 1 tablespoon of cocoa powder for chocolate icing)
Cream the sugar, butter, and salt until light and fluffy.
Add milk and vanilla extract (and chocolate if you are making chocolate icing), beating until well blended.
Test by rubbing a small amount between your fingers, it should not be sandy feeling, if it is blend more till smooth.