PREHEAT oven to 325° F.
COMBINE graham cracker crumbs, butter and sugar in medium bowl; press onto bottom of ungreased 13 x 9-inch baking pan.
BEAT cream cheese, sugar and flour in large mixer bowl until smooth. Gradually beat in evaporated milk, eggs and vanilla extract.
MICROWAVE morsels in medium, uncovered, microwave-safe bowl on HIGH (100%) power for 1 minute; STIR. Morsels may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until morsels are melted. Stir 1 cup cream cheese mixture into chocolate. Pour remaining cream cheese mixture over crust. Pour chocolate mixture over cream cheese mixture. Swirl mixtures with spoon, pulling plain cream cheese mixture up to surface.
BAKE for 40 to 45 minutes or until set. Cool completely in pan on wire rack; refrigerate until firm. Cut into 15 rectangles; cut each rectangle in half diagonally to form triangles.