Butter a 9x5x3-inch loaf pan. Combine sugar, milk, cocoa, corn syrup and salt in two-quart saucepan. Cook over medium heat, stirring constantly until chocolate and sugar are dissolved. Cook, stirring occasionally to 234 degrees on candy thermometer (or until small amount of mixture dropped into very cold water forms a soft ball which flattens when removed from water).
Remove from heat, add butter. Cool mixture without stirring (bottom of pan will be lukewarm). Add vanilla, beat vigorously and continuously 5 to 10 minutes with wooden spoon until candy is thick and no longer glossy. Mixture will hold its shape when dropped from spoon.
Quickly stir in nuts, spread mixture evenly in buttered pan. Cool until firm. Cut into 1-inch squares. Makes 1 pound.