2 cups crispy rice cereal (such as Rice Krispies®)
2 cups salted peanuts
Preheat oven to 325 degrees F (165 degrees C). Grease a 9×13-inch baking dish.
Stir cake mix, 1/3 cup melted butter, and egg together in bowl. Press mixture into prepared baking dish.
Bake in the preheated oven until golden and fragrant, about 15 minutes. Spread marshmallows over crust and return to oven. Bake until marshmallows are melted, 2 to 3 minutes. Cool completely.
Melt peanut butter chips, corn syrup, 1/4 cup butter, and vanilla extract together in a large saucepan over low heat. Stir crispy rice cereal and peanuts into corn syrup mixture. Pour peanut mixture over marshmallows. Cool completely. Chill in refrigerator until hardened, about 1 hour. Cut into 1-inch squares.