PREHEAT oven to 350°F. Coarsely chop 8 of the cookies; set aside. Finely crush remaining 26 cookies; mix with butter. Press crumb mixture firmly onto bottom of greased 9-inch square baking pan.
TOP crust evenly with dollops of marshmallow creme, leaving 1/2-inch border around all sides. Sprinkle with chopped chocolate, walnuts and chopped cookies; press lightly into crust.
BAKE 15 min. or until marshmallow creme just begins to brown. Cool completely in pan on wire rack before cutting into bars to serve.