Line a 8 or 9-inch square pan with foil. Generously grease the foil with butter. Combine the sugar, cocoa powder, and salt in a large, heavy saucepan over medium heat. Mix well. Using a wooden spoon (do not use a whisk), stir in the milk. Cook, stirring constantly, until the milk comes to a full, rolling boil. Let it boil without stirring until it reaches 234 degrees F on a candy thermometer (soft ball stage, drop a small amount in very cold water to test it - it should form a soft ball that flattens when removed from the water). Remove the pan from the heat. Add the butter and vanilla to the saucepan but DO NOT STIR. Just let the mixture sit until it has reached 110-120 degrees F. Once cooled, beat the fudge with the wooden spoon until it thickens and has lost some of its glossiness. Quickly spread the fudge in the greased pan. Let cool slowly (do not rush the cooling process or it may not set up) then cut into squares when completely cooled. Store the fudge wrapped loosely in foil in the refrigerator. cook's notes You can use other flavor extracts in place of vanilla (peppermint, caramel, strawberry, coffee, etc). Add-ins such as dried fruit or nuts can be added as it sets. Do not use margarine as it has a higher water content that can keep the fudge from setting up properly.