Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of 9-inch square pan with shortening or cooking spray. In small bowl, beat all filling ingredients with electric mixer on low speed until smooth. Set aside.
Make brownie batter as directed on box, using ingredients as listed above.
Spread ¾ of the batter in pan. Spoon filling by tablespoonfuls evenly over batter. Spoon remaining brownie batter over filling. Cut through batter several times with knife for marbled design.
Bake 40 to 45 minutes or until toothpick inserted 1 inch from side of pan comes out almost clean. Cool completely. Cut into 4 rows by 4 rows.
Store covered in refrigerator.
If the batter is not spreadable enough for you, add another egg, or a bit more water, or a bit more oil so that the consistency better lends itself to being evenly distributed in the pan.