This is a great buttercream frosting.
From I Scream for Buttercream even though the idiots don't let you copy and paste their recipe like they are something special!

Cream the butter for several minutes at medium speed.
Add half the sugar, beat on low until it is mixed in, then on medium.
Beat in half the milk, then add the rest of the sugar, beating on low until mixed in, then on medium.
Mix in the rest of the milk and the vanilla.
Adjust the thickness by adding more milk to thin it or more sugar to thicken it.
When you have the right consistency, beat on medium for about 5 minutes.