PREPARE and bake cake mix in 2 (9-inch) round cake pans as directed on package . Cool completely.
COARSELY chop 10 of the cookies. Add to 2 cups of the whipped topping in large bowl along with 1-1/2 cups of the marshmallows; stir gently until well blended.
PLACE one cake layer on serving dish; spread with whipped topping mixture. Cover with second cake layer. Frost top with remaining 1 cup whipped topping. Refrigerate until ready to serve. Garnish with remaining 10 cookies and 1 cup marshmallows just before serving.