Heat oven to 350 degrees. Coat 13 x 9 pan lightly with non-stick spray.

Beat cake mix, oil and egg in large bowl on medium speed until mixture resembles coarse crumbs. Stir in nuts. Reserving 1 1/2 cup crumb mixture, press the 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 a corner of 1-quart zip close bag, Microwave the bag for 5 sec, or warm by placing on top of the heating oven. Snip a small corner off the bag, drizzle the frosting in a zig zag pattern over the bars. Chill slightly to set frosting and ease of cutting.