Line an 8-in. square pan with foil and grease the foil with 1-1/2 teaspoons butter; set aside. In a heavy saucepan, combine sugar, cocoa and salt. Stir in the milk until smooth; bring to a rapid boil over medium heat, stirring constantly. Cook without stirring until a candy thermometer reads 234° (soft-ball stage). Remove from the heat; add vanilla and remaining butter (do not stir). , Cool to 110° (about 5 minutes). Beat with a spoon until fudge thickens and just begins to lose its gloss. Immediately spread into prepared pan. Cool. Using foil, lift fudge out of pan. Discard foil; cut fudge into 1-in. squares.