Butter 8 or 9 inch square pan, set aside. In heavy 4 quart saucepan combine sugar, cocoa and salt, stir in milk. Cook over medium heat, stirring constantly until mixture comes to full rolling boil. Boil without stirring to 234*F (soft ball stage) or until syrup when dropped into very cold water forms a soft ball which flattens when removed form water.(Bulb of candy thermometer should not rest on bottom of saucepan). Remove from heat. Add butter and vanilla. DO NOT STIR. Cool at room temperature to 110*F (lukewarm). Beat with wooden spoon until fudge thickens and loses some of its gloss. Quickly spread into prepared pan cool. Cut into squares. Makes about 3 dozen candies