Measure out all ingredients and make sure they are close to your working area on the stove. You can combine the peanuts, salt and baking soda in one medium bowl for ease.
Line a rimmed quarter sheet pan with parchment paper, then spray with nonstick cooking spray.
In a wide, deep, heavy bottomed medium saucepan, combine sugar and water. Stir to moisten the sugar, then set over medium heat and bring to a boil. Do not stir the mixture, but instead swirl the pan as it cooks.
When the sugar reaches a boil, add butter and carefully set a candy thermometer in the pot. Reduce heat to medium low, swirling the pan every once in a while, until the mixture reaches at least 310˚F, or a light caramel color, known as the hard crack stage. Remove the pan from the heat and add the peanut mixture. Carefully stir 4 to 5 times, or until the peanuts are incorporated. Take care during this step as the baking soda will cause the sugar mixture to bubble and expand.
Working quickly, dump the mixture on the prepared pan, spreading with a rubber spatula into an even layer.
Let sit for about 30 minutes to harden, and break into 2 inch pieces. Store in an airtight container for up to 1 week at room temperature or up to 1 month in the freezer.