In a large bowl, combine the cake mix, butter and egg; beat on low speed for 30 seconds. Beat on medium for 2 minutes or until no longer crumbly. Stir in peanut butter cups.
Press into a greased 13-in. x 9-in. baking pan. Bake at 350° for 18-22 minutes or until lightly browned.
Meanwhile, in a large saucepan, combine the cornstarch, caramel topping and peanut butter; stir until smooth. Cook over low heat for 25-27 minutes or until mixture comes to a boil; stirring occasionally. Remove from the heat; stir in peanuts.
Spread evenly over warm crust. Bake 6-7 minutes longer or until almost set. Cool completely on a wire rack. Spread with frosting; sprinkle with peanuts. Cover and refrigerate for at least 1 hour before cutting. Store in the refrigerator. Yield: about 3 dozen.