Preheat oven to 350°F. Grease a 13x9-inch baking dish with cooking spray or butter.
For the base:
In a large bowl, stir together the yellow cake mix, ⅓ cup melted butter, and egg.
Press into the prepared baking dish, and bake for 12–15 minutes, or until lightly golden brown around the edges.
Remove from the oven and sprinkle marshmallows over the top. Bake for an additional 1–2 minutes, or until puffy.
For the topping:
In a saucepan, heat the corn syrup and butter over medium heat until the butter is melted. Remove from the heat and stir in the peanut butter chips and vanilla until smooth.
Stir in the Rice Krispies and peanuts.
Spoon the mixture over the marshmallows and spread carefully (I like using an offset spatula for this).
Refrigerate until firm, 1–2 hours. Remove from the fridge and cut into bars. These are very rich!
Video
Notes
Use Pillsbury or Duncan Hines cake mix. Betty Crocker has decreased the size of their cake mixes by 1–2 ounces, depending on the variety. The bars will keep for a week or two (or longer) at room temperature in an airtight container. You can also freeze them for up to three months.