Combine the sugar, cocoa, and salt in a heavy 4-quart saucepan.
Stir in the milk.
Place over low heat, and cook, stirring constantly, until the sugar is dissolved.
Then continue cooking without stirring to 234°, or until a small amount dropped into cold water forms a soft ball.
Remove from the heat; add the butter and, without stirring, cool to lukewarm (110°).
Add the vanilla and beat until the fudge is thick and loses its shine.
Quickly pour into a lightly buttered 8-inch square pan.
When firm, cut into squares.