Prepare the brownie batter according to the directions on the boxed mix.
Pour the batter into a greased 9 x 13-inch baking dish and bake for 20 to 25 minutes, until a toothpick inserted near the center comes out with a few moist crumbs.
Remove the brownies from the oven and evenly sprinkle the 1/2 cup salted peanuts and 1 cup chopped Reese's peanut butter cups over the top, then return to the oven and bake 4 to 6 minutes more.
While the brownies finish baking, combine the 1 1/2 cups milk chocolate chips, 1 1/2 cups creamy peanut butter, and 1/2 tablespoon butter in a microwave-safe bowl; heat in 30-second intervals, stirring between each, until smooth. Alternatively, melt together in a double boiler.
Stir the 1 1/2 cups Rice Krispies cereal into the melted chocolate-peanut butter mixture until evenly coated.
Remove the brownies from the oven and immediately pour the chocolate-peanut butter-Rice Krispies mixture evenly over the hot brownies, spreading gently if needed.
Refrigerate the brownies for at least 2 hours to set, then slice into squares and serve.