HEAT oven to 350ºF. Coat 13 x 9-inch pan lightly with no-stick cooking spray.
BEAT cake mix, oil and egg in large bowl on medium speed until mixture resembles coarse crumbs. Stir in peanuts. Reserve 1 1/2 cups crumb mixture. Press remainder on bottom of prepared pan.
BEAT sweetened condensed milk and peanut butter in medium bowl until smooth. Spread over prepared crust. Sprinkle with reserved crumb mixture.
BAKE 25 to 30 minutes or until set. Cool. Place frosting into corner of 1-quart resealable food storage bag. Seal bag. Microwave 5 seconds on HIGH. Snip small edge off bag corner. Squeeze bag, drizzling in a criss-cross pattern over crumb topping. Chill to set chocolate drizzle for ease in cutting into bars.