Line 8 or 9 inch square pan with foil, extending foil over edges of pan; butter foil or spray with cooking spray. Mix cocoa, sugar, and salt in heavy 4 quart saucepan, and stir in milk. Cook over medium heat stirring constantly, until mixture comes to full rolling boil. Boil without stirring, until mixture reaches 234 degrees F on candy thermometer or until small amout of mixture dropped into very cold water forms a soft ball which flattens when removed from water. (Bulb of candy thermometer should not rest on the bottom of saucepan.) Remove from heat. Add butter and vanilla. DO NOT STIR! Cool at room temperature to 110 degrees F (lukewarm). Beat with wooden spoon until fudge thickens and just begins to lose some of its gloss. This step may take a while, so be patient! Spread quickly into prepared pan; cool completely. Cut into squares. Makes about 36 pieces of 1 3/4 lbs. For best results, do not double this recipe.