Preheat oven to 350 degrees. Lightly grease a 13 x 9 x 2-inch baking pan. In the bowl of an electric mixer, combine cake mix, egg, and butter and mix well. Pat into the bottom of prepared pan and set aside. Still using an electric mixer, beat cream cheese until smooth; add eggs and vanilla. Dump in confectioners' sugar and beat well. Reduce speed of mixer and slowly pour in butter. Mix well.
Pour filling onto cake mixture and spread evenly. Sprinkle 1 cup chocolate chips on top of filling. Bake for 40 to 50 minutes. Don't be afraid to make a judgment call on the cooking time, because oven temperatures can vary. You want the center to be a little gooey.