Chocolate Butter-Cream Frosting
["1 1/2 c. (3 sticks) butter or margarine, softened", "2 c. confectioners sugar", "3 large egg yolks", "1 c. (6 oz.) semi-sweet chocolate pieces, melted"]
["In a large bowl, beat at high speed and cream butter; gradually add sugar.", "Beat until fluffy.", "Beat in egg yolks one at a time.", "Gradually beat in melted chocolate until creamy.", "If too soft, refrigerate for 1/2 hour."]
["butter", "confectioners sugar", "egg yolks", "semi-sweet chocolate"]