Combine sugar, cocoa, salt and milk in large saucepan over medium heat
Stir constantly until mixture is at full rolling boil
Continue boiling 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 from water.
(NOTE: bulb of candy thermometer should not rest on bottom of saucepan)
Remove from heat
Add butter, vanilla and black walnuts (if desired). 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 buttered 8 or 9 inch square pan.