Prepare your tools. Grease a large baking sheet or line it with parchment paper so your brittle doesn’t stick.
Make the syrup. In a medium saucepan, combine sugar, corn syrup, and ¼ cup water. Cook over medium heat until the mixture reaches 250°F (hard-ball stage).
Add peanuts. Stir in the peanuts and continue cooking until the mixture reaches 300°F (hard-crack stage).
Add butter, salt, and baking soda. Quickly stir in butter, salt, and baking soda. The mixture will foam up – that’s what makes it airy!
Spread and cool. Immediately pour the mixture onto your prepared baking sheet. Use a greased spatula to spread it into a thin layer. Let it cool completely before breaking into pieces.
Notes
Be sure to have all your ingredients ready before starting – this recipe moves quickly once it reaches temperature.