Preheat oven to 325°. Spray an 8-inch square baking pan with cooking spray. Line pan with parchment paper, letting excess extend over sides of pan.
In a medium bowl, whisk together oil, ¼ cup water, and egg. Add brownie mix, and whisk until well combined. Spoon mixture into prepared pan, and spread smooth with a small offset spatula.
Bake until a wooden pick inserted in center comes out with a few moist crumbs, about 40 minutes. Sprinkle 2 cups marshmallows onto brownie, and bake until marshmallows have melted slightly, about 5 minutes more.
In the top of a double boiler, heat peanut butter, chocolate chips, and butter over simmering water, stirring occasionally, until melted and well combined. Add salt, vanilla, and remaining 2¾ cups marshmallows, and stir until marshmallows are melted. Add cereal, stirring until combined. Spoon mixture onto marshmallow layer, and using an offset spatula, lightly spread and press into an even layer. Let stand for at least 2 hours.
Using a serrated knife, cut into bars. Store in airtight container for up to 3 days.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/browniepeanutbuttercrunchbars/