PREPARE and bake brownies in 13x9-inch pan as directed on package; cool. Meanwhile, beat pudding mix and milk with whisk 2 min. Add peanut butter and sugar; mix well. Refrigerate until brownies are completely cooled.
SPREAD over brownies.
MICROWAVE COOL WHIP and chocolate in microwaveable bowl on HIGH 1 min., stirring every 30 sec. Spread over pudding; top with nuts. Refrigerate 1 hour.