Butter 8 x 8 x 2 inch pan. In 3 1/2 quart saucepan, mix sugar with dry gelatin. Add milk, corn syrup, unsweetened chocolate, and butter. Cook over medium heat and, stirring frequently, to 238ºF. on candy thermometer, or until a little in cold water forms soft ball that flattens when removed from water. Remove from heat. Pour into large mixing bowl. Stir in vanilla. Cool 25 minutes. Beat with wooden spoon until candy thickens. Stir in walnuts. Spread in prepared pan. Let cool, then cut into squares. Makes about 2 1/2 pounds.