Preheat oven to 350°F. Spray a 9 x 9-inch pan with cooking spray and set aside. Place brownie mix in a large bowl. Add butter and eggs and mix with an electric mixer on medium speed until just combined.
Spread half of batter in prepared pan, and set aside remaining batter. In a separate bowl, mix cream cheese, sugar and egg until light and fluffy.
Spoon cream cheese mixture over chocolate layer, then add remaining chocolate batter by the spoonful to cover the cream cheese, spreading carefully. Use a knife to swirl the two together.
Bake until a toothpick inserted in the center comes out clean, about 25 minutes. Cool thoroughly before serving.
