What is cupcake icing made of?

There are many different recipes but the general one is confectioners sugar, a dash of salt, butter and vanilla. You substitute cream cheese for the butter for a cream cheese frosting or use a different extract for the vanilla ie.. almond, peppermint. Or you can add cocoa to make a chocolate one. Oh and add a bit of milk, a little at a time, until you get the consistency you want.