Line an 8-inch square pan with buttered parchment paper.

In a medium heavy saucepan, combine sugar, cocoa, and salt. Add the milk and stir using a wooden spoon.

Cook over medium heat, stirring continuously until the mixture reaches a strong boil, about 15 minutes.

Reduce heat to low-medium and continue cooking without stirring until the temperature reaches 234°F (112°C), approximately 30 minutes.

Remove the pot from heat, then add the butter and vanilla without stirring. Let it cool to room temperature (110°F/43°C), about 30 minutes.

Beat the mixture with a wooden spoon just until it starts losing its gloss, around 6-8 minutes.

Quickly spread the fudge in the prepared cake pan and let it cool completely, even overnight at room temperature. Cut into squares.

Store the fudge in an airtight container at room temperature. It can also be frozen by wrapping it well in a double bag. Freeze it in pieces or as a whole slab for up to 3 months. It keeps in an airtight container for about 2 weeks at room temperature or 3-4 weeks in the fridge.