Line 13x9-inch pan with foil, with ends of foil extending over sides; spray with cooking spray. Prepare cake batter as directed on package, adding 1 pkg. dry pudding mix; beat 2 min. Pour into prepared pan. Bake 25 min. or until toothpick inserted in center comes out clean. Cool completely.
Meanwhile, prepare filling, frosting and chocolate curls. For filling, beat remaining pudding mix and milk in medium bowl with whisk 2 min. Stir in peanut butter until blended. For frosting, microwave 3 oz. chocolate in large microwaveable bowl on HIGH 1 min. or until melted, stirring after 30 sec. Add cream cheese and butter; beat with mixer until blended. Gradually beat in sugar. For chocolate curls, see Tip.
Use foil handles to lift cake from pan; cut crosswise in half. Place 1 cake half, top-side down, on plate; spread with peanut butter filling. Cover with remaining cake half, top-side up. Spread top and sides with chocolate frosting. Press nuts halfway up sides of cake; top with chocolate curls.