Prepare the brownie mix according to package directions, using a 13x9-inch baking pan and the directions for chewy(not cake-like) brownies. Bake according to directions.
Sprinkle baked brownies with marshmallows and return to oven for 3 to 5 minutes or until golden. Cool in the pan on a wire rack.
In a saucepan, combine chocolate chips, peanut butter, and butter. Heat and stir until the chocolate is melted. Remove from heat; stir in the cereal. Pour evenly over marshmallows.
Refrigerate until topping is set. Cut into bars. Serve at room temperature.