Make brownie batter as directed on box, adding reserved egg yolk. Reserve 1/2 cup brownie batter; set aside. Spread remaining batter in pan. Spoon filling by tablespoonful dollops evenly onto batter, making 3 rows by 3 rows. Spoon about 1 tablespoon of the reserved brownie batter in center of each filling dollop. Draw knife through mixture in four straight lines horizontally, then vertically for swirled design. Sprinkle with chocolate chips.