LINE a 13 x 9-inch baking pan with aluminum foil; spray with no-stick cooking spray. Prepare brownie mix according to package directions; stir in walnuts. Turn into prepared pan. Bake as directed. Cool thoroughly.
COMBINE sweetened condensed milk, peppermint extract and food coloring, if desired, in large bowl. Fold in whipped cream and chocolate chips. Spoon over brownie layer.
COVER; freeze 6 hours or until firm. To serve, lift from pan with foil; cut into squares. Serve with hot fudge sauce, if desired.