Line 9-inch square pan with foil, with ends of foil extending over sides of pan. Arrange grahams on bottom of pan; set aside.
Mix 2 cups of the marshmallows, the chocolate and milk in microwaveable bowl. Microwave on HIGH 1 min.; stir. Microwave on HIGH 30 sec.; stir until chocolate is completely melted. Cool.
Add whipped topping, remaining 1/2 cup marshmallows, the chopped cookies and walnuts to marshmallow mixture; stir gently until well blended. Carefully spread over grahams in prepared pan.
Freeze several hours or until firm.
Remove dessert from pan, using foil handles.
Cut dessert into 12 bars to serve. Store any leftover bars in freezer.
