
Combine the peanuts, salt, and baking soda in a small bowl. Measure the butter and set it next to the peanut mixture.
Line a rimmed quarter sheet pan with parchment paper.
In a heavy-bottomed medium saucepan, combine the sugar and water. Stir to moisten the sugar, then set over medium heat and bring to a boil. Do not stir the mixture; instead, swirl the pan as it cooks.
When the sugar reaches a boil, add the butter and carefully set a candy thermometer in the pot. Reduce the heat to medium-low, swirling the pan every once in a while until the mixture reaches between 300˚F and 310˚F, or a 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 to incorporate the peanuts. Take care during this step, as the baking soda will cause the sugar mixture to bubble and expand.
Working quickly, dump the mixture onto the prepared pan, spreading it into an even layer with a rubber spatula.
Let the peanut brittle sit for about 30 minutes to harden, then break into 2-inch pieces and enjoy.