From The Cake Mix Doctor which has some really terrific frosting recipes. Never, ever use frosting from a can.
Chocolate Buttercream Frosting
8 tablespoons butter, room temperature
2/3 cup unsweetened cocoa powder
3 cups powdered sugar
1/3 cup milk
2 teaspoons vanilla
1/4 teaspoons salt
Beat the butter and cocoa powder until well combined. Add remaining ingredients and beat until frosting lightens and is fluffy. Add additional milk or sugar if necessary to achieve desired consistency.