Butter an 8- or 9-inch square baking pan, or line pan with foil, extending foil over edges of pan to create handles. Butter the foil; set pan aside.

Combine sugar, salt, butter, evaporated milk and marshmallow fluff in a heavy-bottomed 3-quart sauce pan set over low heat, stirring until well blended. Increase heat to heat to medium-high and bring mixture to a full-rolling boil being careful not to mistake escaping air bubbles for boiling. As soon as the mixture reaches a boil, lower heat to medium and boil slowly, stirring constantly, for 5 minutes, or until mixture reaches soft ball stage (between 234 and 240 degrees F). Remove pan from heat, add vanilla and chocolate pieces, stirring until chocolate is melted and well combined. Stir in nuts, if desired. Turn mixture into prepared pan and let cool at room temperature on wire rake until firm, about 4 hours. Remove fudge from pan using foil handles and cut into thirtysix 1 1/3-inch to 1 1/2-inch wide squares depending on size of pan used. Store in an airtight container (layers seperated by wax paper) for up to a week.

Makes about 2 1/2 pounds of fudge for 36 servings.