1/2 cup butter or margarine
1/3 cup milk
1/4 cup cocoa
1 (1-pound) package powdered sugar
1 teaspoon vanilla extract
Melt butter in a saucepan over medium heat. Whisk in milk and cocoa, and bring mixture to a boil. Remove from heat. Gradually add powdered sugar, stirring until smooth; stir in vanilla extract.
NOTE: To thin frosting, add 1 to tablespoons milk.