MICROWAVE chocolate in large microwaveable bowl on HIGH 2 min. or until chocolate is almost melted, stirring after 1 min. Stir until chocolate is completely melted. Cool 5 min. or to room temperature.
ADD sugar, butter and vanilla; beat with electric mixer on low speed until well blended.
GRADUALLY add milk, beating until well blended. If frosting becomes too thick, beat in additional milk by teaspoonfuls until of desired spreading consistency. Use to frost your favorite cake or cupcakes.