Line an 8-inch square baking pan with foil, then grease the foil with 1-1/2 teaspoons of butter to ensure easy removal of the fudge once set.

In a heavy saucepan, mix sugar, cocoa powder, and salt. Gradually add milk, stirring until smooth.

Set saucepan over medium heat, stirring constantly. Bring to a rapid boil while stirring to prevent sticking.

Insert a candy thermometer and cook without stirring until the mixture reaches 234°F (soft-ball stage).

Remove from heat. Add the remaining butter and vanilla but do not stir. Let cool undisturbed to 110°F.

Once cooled, beat with a wooden spoon until it thickens and loses its glossy shine.

Spread the fudge evenly in the prepared pan and let cool completely. Cut into 1-inch squares to serve.