Start by lining a baking sheet with parchment paper. This will be where you pour your finished brittle to cool.
In a heavy-bottomed saucepan, combine the granulated sugar, water, and light corn syrup. Stir gently to combine.
Place the saucepan over medium heat and bring the mixture to a boil. Avoid stirring once it starts to boil to prevent crystallization.
Insert your candy thermometer into the mixture, ensuring it does not touch the bottom of the pan. Continue boiling until the temperature reaches 300°F (149°C), known as the "hard crack" stage.
Once the mixture reaches the desired temperature, quickly add the salted dry-roasted peanuts, kosher salt, and unsalted butter. Stir continuously until the butter is melted and the peanuts are well coated.
Remove the pan from heat and carefully stir in the baking soda. The mixture will bubble up, so be cautious!
Immediately pour the hot mixture onto your prepared baking sheet. Use a spatula to spread it out evenly, working quickly as the mixture will begin to set.
Allow the peanut brittle to cool completely at room temperature. Once hardened, break it into pieces and enjoy!
