Line an 8x8 inch square pan with parchment paper, leaving an overhang on the sides to lift the fudge out easily after it sets. Lightly grease the parchment paper with butter or non-stick spray.

In a heavy-bottomed saucepan, whisk together the sugar, cocoa powder, and salt until thoroughly combined and free of lumps.

Stir in the milk until the dry ingredients are moistened. Place the saucepan over medium heat.

Cook the mixture, stirring constantly, until it comes to a full boil. Once boiling, reduce the heat to low and continue to simmer without stirring until the mixture reaches the soft ball stage (235°F to 240°F on a candy thermometer), which should take about 5-10 minutes. Avoid stirring during this stage as it can cause the fudge to become grainy.

Remove the saucepan from the heat and add the butter and vanilla extract on top of the fudge mixture, but do not stir. Allow the mixture to cool to about 110°F, or until the pan is warm to the touch, not hot.

Once cooled, use a wooden spoon to beat the fudge vigorously until it thickens and starts to lose its gloss. This process may take some time and effort but is crucial for achieving the right texture.

Quickly pour the thickened fudge into the prepared pan and smooth the top with a spatula.

Allow the fudge to set at room temperature. This can take several hours, but you can expedite the process by placing it in the refrigerator.

Once set, use the parchment paper overhang to lift the fudge out of the pan. Cut into small squares using a sharp knife. If the knife sticks, dipping it in hot water before each cut can help.