HEAT sugar, butter or margarine, evaporated milk and marshmallow creme in heavy saucepan over medium-high heat until mixture boils, stirring constantly. Boil and stir for 5 minutes.

ADD chocolate to marshmallow mixture; stir until melted. Add walnuts.

POUR immediately into foil-lined 9-inch square pan. Spread evenly to cover bottom of pan. Refrigerate 4 hours or overnight. Cut into squares. Store in airtight container in refrigerator.