Beat cake mix and butter until combined. Add egg and beat until well mixed. Press into the bottom of an ungreased 13×9 baking pan.
Bake until it sets and the edges are lightly browned (about 15 minutes). Remove from oven and immediately sprinkle marshmallows over hot crust.
Bake another couple of minutes until marshmallows puff up. Remove from oven.
Combine corn syrup, sugar and brown sugar in saucepan. Bring to a boil over medium heat, stirring constantly. Remove from heat. Add in peanut butter and vanilla and stir until smooth. Stir in cereal and peanuts.
Spoon over marshmallow topped base and spread to cover. Sprinkle candy corn over mixture evenly and gently press in.