Line your 8x8 inch baking pan with parchment paper or wax paper, letting the edges hang over the sides for easy removal later.

In your heavy saucepan, whisk together the sugar and cocoa powder thoroughly, breaking up any lumps in the cocoa.

Pour the milk into the sugar and cocoa mixture, stirring constantly with a wooden spoon until you have a smooth paste with no dry streaks.

Place the saucepan over medium heat and add the butter, stirring occasionally as everything melts together. Once the mixture is smooth and the butter is fully incorporated, stop stirring and let it bubble gently for about 8 to 10 minutes without touching it.

Test for doneness by dropping a small amount of hot fudge into your bowl of cold water; it should form a soft ball that holds together but still gives slightly when you press it (236 to 238 degrees Fahrenheit). If it's still too soft and dissolves, let it cook another minute and test again.

Remove the saucepan from heat and let it cool for about 3 to 5 minutes without stirring. Add the vanilla extract and salt, stirring very gently just a few times to distribute them throughout.

Using your wooden spoon, stir the fudge vigorously for about 5 to 7 minutes until it thickens, loses its glossy shine, and starts to look slightly granular at the edges.

Quickly pour the fudge into your prepared pan, using your spatula to smooth the top into an even layer. Let it cool at room temperature for at least 30 minutes, then refrigerate for another 15 minutes to fully set before cutting.