Preheat oven to 350 degrees. Grease a 9 x 13 baking dish.
In a mixing bowl, mix together dry cake mix, egg and melted butter. Beat until well combined. Your batter will be thick.
Spread batter evenly on the bottom of baking dish. Set aside.
FOR FILLING:
Beat cream cheese, eggs, melted butter and vanilla until smooth. Gradually add powdered sugar. Beat until fully combined.
Remove 1 cup of cream cheese mixture. Set aside.
Spread remainder of cream cheese mixture over cake batter base.
In your microwave or in a saucepan, melt chocolate chips until smooth. Mix melted chocolate chips with reserved 1 cup of cream cheese mixture. Mix until fully combined.
Spoon dollops of chocolate cream mixture on top of plain cream cheese batter.Swirl carefully with a butter knife, creating a marble effect.
Bake for 45 minutes. Don’t let cake bake too long..it should be soft.