Prepare the brownies as directed on the box in a 9 x 13” pan.
After the brownies have been baked, sprinkle them with the marshmallows and return to the oven for 3-5 minutes. Allow to cool in the pan on a rack.
Combine the chocolate chips, peanut butter, and butter in a saucepan. Melt, stirring constantly, over medium heat.
Add the Rice Krispies to the melted chocolate/peanut butter mix and spread evenly over the marshmallow layer on the brownies. Cool and serve at room temperature.