Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Mix brownie mix, oil, water, and eggs in a large bowl until smooth; spread into prepared baking dish.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 28 to 30 minutes. Sprinkle marshmallows over brownie and continue baking until slightly melted, about 3 minutes more. Remove from oven and cool completely.
Heat chocolate chips, peanut butter, and butter in a saucepan over low heat; cook and stir until smooth. Remove from heat and stir in rice cereal. Spread over cooled brownies. Refrigerate until firm; slice brownies to serve.
