Mix cake according to package directions and pour into 9 x 13 inch pan. Bake accordingly. While cake is still warm, frost with topping.
Almost when cake is done baking, mix the butter, sugar, milk and peanut butter till blended thoroughly. Stir in peanuts.
Spread topping on warm cake evenly. Set oven to broil and place cae 5 inches from heat. Broil for about 3 minutes or until topping bubbles and is slightly browned. Be careful and this will burn quickly!