Coat a 15x10x1-inch baking pan with cooking spray; set aside.
Prepare brownie mix according to package directions. Spread evenly in the prepared pan.
In a medium mixing bowl, beat cream cheese with an electric mixer for 30 seconds. Gradually beat in the powdered sugar and milk until well combined. Add the cake mix and water. Beat well. Spoon the mixture atop brownie batter. Gently lift and fold brownie batter to marble into the white batter.
Bake in a 350 degree F oven for 25 to 30 minutes or until toothpick inserted near center comes out clean. Cool in pan on a wire rack. Using a wet knife, cut into bars. Store in refrigerator. Makes 48 brownies.