Preheat oven to 325°.
Prepare brownies as directed on box, using the recommended oil, water and eggs for fudgy brownies. Spread batter in to a 13"x9" pan sprayed with non-stick cooking spray or lined with foil.
In a large mixing bowl, beat together cream cheese, sugar, egg yolk, and vanilla just until smooth.
Put dollops of the cheesecake batter on top of the brownie batter and use a knife to swirl some of the brownie batter in to the cheesecake batter. Sprinkle chocolate chips across the top.
Bake 50 - 55 minutes until edges are pulling away from the side of the pan, and the middle is set but not firm. Allow to cool completely in the pan before serving.