HEAT over to 325ºF. Coat a 15 x 10-inch jelly roll pan with no-stick cooking spray.
PREPARE cake mix batter according to package directions using water, oil and eggs. Spread in prepared pan. Bake 16 to 20 minutes or until toothpick inserted in center comes out clean. Cool completely in pan on wire rack.
BEAT milk and pudding mix in large bowl with electric mixer on medium speed until thickened, about 15 seconds. Add peanut butter; beat until smooth. Beat in whipped topping until smooth. Spread over cooled cake. Garnish with chopped peanuts.