1Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of 13x9-inch pan with shortening or cooking spray. (For easier cutting, line pan with foil, then grease bottom only of foil.) Make brownies as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
2In medium bowl, beat Filling ingredients with electric mixer on medium speed until smooth. Spread evenly over brownie base.
3In 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 9 rows by 4 rows. Store covered in refrigerator.