In a heavy 2 quart saucepan, over medium heat, bring to a boil sugar, corn syrup, salt, and water. Stir until sugar is dissolved. Stir in peanuts. Set candy thermometer in place, and continue cooking. Stir frequently until temperature reaches 300 degrees F (150 degrees C), or until a small amount of mixture dropped into very cold water separates into hard and brittle threads.
Remove from heat; immediately stir in butter and baking soda; pour at once onto cookie sheet. With 2 forks, lift and pull peanut mixture into rectangle about 14x12 inches; cool. Snap candy into pieces.