Preheat oven to 350°F. Combine the butter and cake mix in large bowl. Beat on low speed until well mixed. Add egg; continue beating until well mixed.
Press onto bottom of a parchment lined or greased 15x10x1-inch jelly-roll pan. It will be a really thin layer. I found that using the heel of my hand I could easily press it evenly across the pan. Bake for 8-10 minutes or until set, it will not be brown. Remove from oven.
Immediately sprinkle marshmallows over hot, partially baked crust. Return to oven; continue baking for 1 to 2 minutes or until marshmallows puff.
For the topping: Make sure you have all the ingredients ready to go because the mixture sets quickly. Combine corn syrup, sugar and brown sugar in large saucepan. Cook over medium heat, stirring until mixture comes to a boil (2 to 4 minutes). Remove from heat; stir in peanut butter and vanilla until smooth. Quickly stir in cereal and peanuts. Mixture with be thick. Immediately spoon over marshmallows and spread to cover. I pressed the mixture down into the marshmallow. Quickly sprinkle the candy corns on top and press them down into the bars. Cool completely. Cut into small bars.