PREHEAT oven to 350°F. Pour milk into large bowl. Add pudding mix. Beat with wire whisk 2 minutes.
SPREAD graham pieces in lightly greased 9-inch square baking dish. Pour pudding mixture over grahams.
BAKE 20 minutes. Sprinkle evenly with marshmallows and chocolate chunks. Bake an additional 10 minutes. Remove from oven. Let stand 10 minutes before serving.