In medium bowl, beat cream cheese and butter with electric mixer on medium speed until well blended. Add egg; beat well. Stir in cake mix by hand until well combined (mixture will be thick). With hands buttered or sprayed with cooking spray, press about 2 1/4 cups cake mixture in pan. Pour condensed milk on top of cake mixture. Top with crushed graham crackers, marshmallow halves and half of the chocolate pieces. Drop remaining cake mixture by spoonfuls on top.