Prepare the Pan: Line an 8- or 9-inch square pan with foil, taking the foil up over the sides, and butter it generously.
Mix Dry Ingredients: In a heavy saucepan, combine sugar, cocoa powder, and salt. Mix thoroughly so no cocoa clumps remain and everything is evenly distributed.
Add Milk: Stir in the milk gradually using a wooden spoon.
Cook & Stir: Place the saucepan over medium heat. Stir constantly until the mixture reaches a full, rolling boil.
Boil without Stirring: Once the boiling point is reached, stop stirring. Continue to boil until the temperature reaches 234°F (soft ball stage).
Add Butter and Vanilla: Immediately remove the pan from heat. Add melted butter and vanilla extract, but do not stir. Allow the mixture to cool at room temperature undisturbed.
Beat the Fudge: Once cool, add black walnuts if using. Beat the fudge with a wooden spoon until it thickens and loses some of its gloss.
Shape and Cool: Immediately spread the mixture into the prepared pan or plate. Smooth out quickly as it begins to set fast.
Cut into Squares: After fully cooling, cut into squares. Use a warm knife for clean, even cuts.
Storage: Wrap loosely in foil and refrigerate, or leave at room temperature in a covered tin.
