Stir together the sugar, cocoa and salt. Then add the milk and stir with a wooden spoon.
Turn heat to low medium and continue to cook without stirring until temperature reaches 234 °F (112C) Approximately 30 minutes.
Remove pot from the heat, add the butter and vanilla. Do not stir.
Then beat with a wooden spoon just until mixture begins to lose some of it’s gloss.
Spread the fudge quickly in the prepared pan and let cool completely, even overnight (at room temperature). Cut it into squares.