Line a rimmed baking sheet with foil and grease with nonstick cooking spray.
Stir together the sugar, corn syrup and 1/2 cup water in a medium saucepan fitted with a candy/deep-fry thermometer. Bring the mixture to a boil over medium heat, then continue to boil until it's golden brown and it reaches 340 degrees F, about 15 minutes.
Working quickly, remove the pan from the heat and stir in the butter and baking soda until smooth (it will foam and bubble up). Stir in the peanuts until evenly distributed amongst the sugar mixture.
Pour onto the prepared baking sheet and quickly smooth with a rubber spatula into a thin and even layer. Let harden uncovered at room temperature until cooled completely. Break into pieces and store in an airtight container.
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