Line an 8-inch square baking dish with parchment paper or aluminum foil, leaving some overhang.

In a medium saucepan, combine the sugar, cocoa powder, milk, butter, and salt. Stir constantly over medium heat until the mixture is well-mixed and the butter has melted.

Raise the heat slightly and bring the mixture to a boil, stirring continually. Boil for 4-5 minutes without stirring.

Test for the soft ball stage by dropping a spoonful of fudge into a bowl of cold water. If it forms a soft ball, it's ready.

Remove from heat and stir in the vanilla extract. Let it cool for a minute or two.

Pour the fudge into the prepared baking dish and spread it into an even layer. Let it cool completely at room temperature or refrigerate to speed up the process.

Once set (about 2 hours), lift the fudge out using the parchment paper overhang and cut into squares.