Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of 8-inch or 9-inch square pan with cooking spray or shortening. (For easier cutting, line pan with foil, then grease foil on bottom only of pan.) Make brownies as directed on box. Cool completely, about 1 hour.
In medium bowl, beat filling ingredients with electric mixer on medium speed until smooth. Spread mixture evenly over brownie base.
In small microwavable bowl, microwave topping ingredients uncovered on High 30 to 60 seconds; stir until smooth. Cool 10 minutes; spread over filling. Refrigerate about 30 minutes or until set. For brownies, cut into 6 rows by 4 rows. Store your Peanut Butter Truffle Brownies, covered, in the refrigerator.