Line an 8 inch square pan with buttered parchment paper.
In a medium heavy saucepan stir together sugar, cocoa and salt, then add the milk and stir with a wooden spoon (not a whisk or metal spoon).
Cook over medium heat stirring continuously until mixture starts to boil (a strong boil), approximately 15 minutes.
Turn heat to low and continue to cook without stirring until temperature reaches 234 °F. Approximately 30 minutes.
Remove pot from heat, add the butter and vanilla. Do not stir. Cool to room temperature (110°F), approximately 30 minutes. Then beat with a wooden spoon just until mixture begins to lose some of it's gloss (approximately 6-8 minutes).
Then spread quickly in prepared cake pan and let cool completely, even overnight (at room temperature). Cut into squares. Store in an airtight container at room temperature.
You can also freeze the fudge in a double bag, well wrapped. Freeze either in pieces or the whole slab. Can be frozen up to 3 months. Keeps in an airtight container for approximately 2 weeks or in the fridge for 3-4 weeks. Enjoy!