Heat oven to 350. Spray a 9" x 13" baking dish w/ cooking spray. Make brownies as directed on box. Bake as directed on box, let cool.
In bowl, w/ mixer on medium, beat together 1/2 cup butter, peanut butter, powdered sugar and milk until smooth. Spread over cooled brownies.
In microwave-safe bowl, microwave chocolate chips and 1/4 cup butter on high for 30-60 seconds. Stir until smooth. Cool 10 mins. Spread chocolate mix over peanut butter frosting. refrigerate until set, about 30 mins.