Preheat oven to 350 degrees. Grease and flour (or use non-stick spray) a 13 X 9 pan.
Reserve 1 cup dry cake mix and swirl pouch.
In a large bowl combine remaining cake mix, oil and one egg until soft dough forms. Press in bottom and 1 1/2 inches up sides of prepared pan. Bake at 350 degrees for 10 minutes. (Crust will not be brown when done).
In another bowl combine reserved 1 cup cake mix, milk, sugar, sour cream, cream cheese, egg and vanilla, mis at low speed until blended. Beat another 3 minutes at medium or until creamy and smooth. Pour 2/3 of filling over warm crust. To remaining filling add swirl pouch and mis at low speed until well blended.
Spoon chocolate mixture radomly over cheese mixture and pull knife through batter in wide curves then turn pan and repeat to create a swirl pattern.
Bake at 350 degrees for 35-45 minutes or until center is set. Cool completely. Store in refrigerator.