LINE 8-inch square pan with foil, with ends of foil extending over sides. Microwave first 3 ingredients in large microwaveable bowl on HIGH 2 min.; stir until chocolate is completely melted and mixture is well blended.
ADD dry pudding mix; stir with whisk 2 min. Gradually stir in sugar; press onto bottom of prepared pan. Top with nuts; press gently into fudge to secure.
REFRIGERATE 2 hours or until firm. Use foil handles to lift fudge from pan before cutting into pieces.