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.