In large heavy saucepan, combine sugar, corn syrup, water, butter and salt.
Cook over medium heat, stirring until sugar is dissolved.
Cover and cook over medium-low heat 5 minutes.
Uncover and cook, without stirring, over medium heat until 300Â°F registers on candy thermometer (hard-crack stage).
Meanwhile, arrange walnuts in shallow pan and toast lightly in 300Â°F oven.
When candy reaches hard-crack stage, quickly stir in warm walnuts and baking soda.
Turn at once into buttered 15x10x1-inch jelly-roll pan and spread thin.
Let stand until cooled, then break into pieces.