HEAT oven to 350ºF.
PREPARE cake batter as directed on package; blend in 3/4 cup peanut butter, dry pudding mix and milk. Spoon into 24 paper-lined muffin cups.
BAKE 21 min. or until toothpick inserted in centers comes out clean. Cool completely.
BEAT remaining peanut butter, cream cheese and sugar until blended; spoon into resealable plastic bag. Cut small piece from one bottom corner of bag.
CUT cone-shaped piece from top of each cupcake; set removed piece aside. Pipe filling onto cupcakes; replace tops.
MICROWAVE COOL WHIP and bittersweet chocolate in microwaveable bowl on HIGH 2 min. or until chocolate is completely melted and mixture is well blended, stirring every 30 sec. Drizzle over cupcakes. Sprinkle with nuts.