Preheat oven to 350 degrees F. and spray a 9x13 inch metal pan with flour-added baking spray.
Make the white filling first. In a mixing bowl, beat together the cream cheese, butter, sugar, cornstarch, milk and vanilla until creamy and smooth. Add the egg and beat just until mixed. Set aside.
Prepare the cake mix using the directions on the side of the cake mix box. Pour half of the cake mix into the pan, spoon the white mixture as evenly as possible over the top, the pour the rest of the cake batter over the white. Bake on center rack for 30 to 33 minutes (or time given on the box) or until cake appears set. Remove from the oven and let cool completely.
Make the chocolate frosting. In a large mixing bowl, mix the butter, sour cream, sugar, cocoa, salt and 2 tablespoons of the cream as well as you can, then beat with a hand-held mixer until pasty and thick. Continue adding remaining cream until you get a nice, smooth, spreading consistency. Spread over the cake and chill.