In a saucepan over medium heat, melt chocolate chips with milk and salt; cook and stir until thickened, about 25 minutes. Remove from the heat; set aside. Combine wafer crumbs and butter; set aside 1 cup. Press remaining crumbs into a greased 13-in. x 9-in. x 2-in. pan. Chill for 10-15 minutes. Pour chocolate over crumbs. Cover and freeze for 10-15 minutes or until firm. Spread the ice cream over chocolate. Sprinkle with reserved crumbs. Freeze at least 2 hours before serving.