In a heavy saucepan, combine sugar, corn syrup, salt and water. Bring to a boil over MEDIUM heat, stirring until sugar has dissolved, about 10 minutes.
Stir in peanuts. Continue cooking, stirring frequently so the peanuts move around (they can brown too much if they sit at the bottom of the pan), until temperature on the candy thermometer reaches 300 degrees (hard-crack stage).
Remove from heat and stir in the butter. Add baking soda and stir, then immediately pour onto prepared cookie sheet. Spread with rubber spatula.
Allow candy to cool completely then break into pieces.
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Notes
Measure out all the ingredients first. This is crucial as you won't have time to measure things as you go along. This recipe requires that you work quickly when the candy is ready.