In a large mixing bowl, combine the cake mix, melted butter, and egg; beat with an electric hand-held mixer on medium speed until crumbly (or cut in with a pastry blender until evenly mixed and crumbly). Stir in chopped peanuts.
Butter a 13x9x2-inch baking pan. Reserve 1 1/2 cups of the crumbs and press remaining crumbs over the bottom of the prepared baking pan.
In another bowl, combine sweetened condensed milk with the peanut butter until well blended. Spread peanut butter mixture over the crust layer. Sprinkle with the remaining reserved crumbs. Bake at 350° for 25 to 30 minutes, or until set. Cool before serving. Store loosely covered at room temperature.