Line 8 or 9-inch square pan with foil, extending foil over edges of pan. Butter foil. In heavy 4 qt. saucepan, stir together sugar, cocoa and salt; stir in milk. Cook over medium heat, stirring constantly, until mixture comes to a full rolling boil. Boil, without stirring, until mixture reaches 234ø on candy thermometer or until syrup, when dropped into very cold water, forms a soft ball which flattens when removed from water. (Bulb of thermometer should not rest on bottom of saucepan).
Remove from heat. Add butter and vanilla. (DO NOT STIR). Cool at room temperature to lukewarm. Beat with wooden spoon until fudge thickens and just begins to lose some of its gloss. Quickly spread into prepared pan; cool completely. Use foil to lift fudge out of pan, peel off foil. Cut into squares. Store tightly covered at room temperature. Approx. 3 doz. pieces or 1-3/4 lbs.