1Grease bottom and sides of 8-inch square pan with butter.
2In 3-quart saucepan, cook sugar, milk, corn syrup, salt and chocolate over medium heat, stirring constantly, until chocolate is melted and sugar is dissolved. Cook, stirring occasionally, to 234°F on candy thermometer or until small amount of mixture dropped into cup of very cold water forms a soft ball that flattens when removed from water; remove from heat. Stir in butter.
3Cool mixture without stirring to 120°F, about 1 hour. (Bottom of saucepan will be lukewarm.) Add vanilla. Beat vigorously and continuously 5 to 10 minutes, using wooden spoon, until mixture is thick and no longer glossy. (Mixture will hold its shape when dropped from a spoon.)
4Quickly stir in nuts. Spread in pan. Let stand about 1 hour or until firm. Cut into 1-inch squares.