HEAT oven to 350°F.
LINE 13x9-inch pan with foil, with ends of foil extending over sides. Crush 14 grahams. Mix with butter and 3 Tbsp. sugar; press onto bottom of prepared pan. Coarsely chop remaining 4 grahams.
BEAT cream cheese, 1 cup sugar and vanilla in large bowl with mixer until well blended. Add flour; mix well. Add eggs, 1 at a time, mixing on low speed after each just until blended. Chop 1/2 cup chocolate chunks; stir into cream cheese mixture. Pour over crust. Sprinkle with remaining chocolate chunks, marshmallows and chopped grahams.
BAKE 40 min. or until center is almost set. Cool completely. Refrigerate 4 hours. Use foil handles to lift cheesecake from pan before cutting into bars.