Butter a shallow dish and set aside. Combine sugar, salt and milk in a large saucepan. Add chocolate and cook over low heat until chocolate is melted. Stirring occasionally, increase heat and bring to full rolling boil. Continue cooking until it reaches soft ball, 110ºC on your candy thermometer. Remove from heat and add butter. Let cool to about 130 degrees. Add vanilla and beat until fudge begins to thicken. RAPIDLY fold in walnuts and cherries (if desired), and INSTANTLY turn the mixture into buttered dish. It's ready to eat almost immediately. Cut into squares and serve.