1/2 cup butter or margarine
3/4 cup half-and-half
6 (1-ounce) squares unsweetened chocolate
5 cups sifted powdered sugar
2 teaspoons vanilla extract
Combine butter, half-and-half, and chocolate in top of double boiler; cook over medium heat, stirring frequently, just until mixture comes to a boil. Remove from heat, and add sugar; beat until smooth. Stir in vanilla.