HEAT oven to 350ºF.
LINE 13x9-inch pan with foil, with ends of foil extending over sides; spray with cooking spray. Arrange graham squares in single layer on bottom of pan, overlapping slightly if necessary.
PREPARE brownie batter as directed on package; stir in chopped chocolate, marshmallows and 3/4 cup chopped cookies. Spread over grahams; sprinkle with remaining cookies.
BAKE 27 to 30 min. or until toothpick inserted 1 inch from edge of pan comes out clean. Cool completely. Use foil handles to lift brownie from pan before cutting to serve.