Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch baking dish with cooking spray.
Mix cake mix, 1/2 cup melted margarine, and egg together in a bowl; press into the bottom of the prepared baking dish.
Bake in the preheated oven for 15 minutes. Top cake with marshmallows and return to oven until marshmallows melt, about 5 minutes.
Place 1/4 cup margarine in a large microwave-safe bowl; heat until melted, about 30 seconds. Stir corn syrup into melted margarine until smooth. Add peanut butter chips and heat in microwave until chips are melted, 30 seconds to 1 minute. Stir peanuts and rice cereal into mixture and spread on top of marshmallow layer on cake. Cool completely before cutting into bars.
