Line an 8x8 inch baking pan with parchment paper, leaving edges overhanging for easy removal.

Place butter and milk into a heavy-bottomed saucepan over medium heat and stir until melted and combined.

Add sugar and cocoa powder, stirring until no dry powder remains visible.

Attach a candy thermometer to the side of the pan, ensuring the bulb does not touch the bottom.

Increase heat to medium-high and bring the mixture to a boil without stirring.

When the mixture reaches 238°F (soft ball stage), remove the pan from heat immediately.

Let the mixture cool undisturbed until it reaches 110°F.

Add vanilla extract and salt, stirring gently just a few times to combine.

Beat vigorously with a wooden spoon for 3 to 4 minutes until the mixture thickens and becomes matte.

Pour the fudge into the prepared pan, spreading evenly, and let chill at room temperature for 2 hours or refrigerate for 30 minutes until firm.

Cut into 16 squares using a sharp knife, wiping the blade between cuts for clean edges.