Butter pan.
Heat sugar, milk, chocolate, corn syrup and salt in 3 quart saucepan over medium heat, stirring constantly until chocolate is melted ad sugar is dissolved.
Cook, stirring occasionally, to 234° on candy thermometer or until a small amount of mixture dropped into very cold water forms a soft ball that flattens when removed from water.
Remove from heat; add margarine.
Cool, without stirring to 120° (bottom of pan will be lukewarm). Add vanilla; beat continuously with wooden spoon until candy is thick and no longer glossy, 5 to 10 minutes (mixture will hold its shape when dropped from spoon).
Quickly stir in nuts. Spread in pan. Cool until firm. Cut into 1 inch squares.