Combine graham cracker crumbs, walnuts, and butter, mixing well; press firmly into a buttered 9-inch pieplate. Bake at 375 degrees for 8 to 10 minutes; cool. Spoon coffee ice cream into cooled crust, and spread evenly; freeze until almost firm. Spread vanilla ice cream over coffee layer and freeze until firm. To serve, slice and top with warm BUTTERSCOTCH SAUCE Melt butter in a heavy saucepan over low heat; add brown sugar, and cook 5 to 8 minutes, stirring constantly. Remove from heat and gradually stir in half-and-half. Cook 1 minute, and remove from heat. Stir ----