1. Line a rimmed baking sheet with a silicone baking mat or parchment paper that has been coated with non-stick cooking spray; set aside.
2. In a medium saucepan, combine sugar, water, corn syrup and salt. Bring to a rapid simmer over medium-high and cook until deep golden, about 20 to 25 minutes. Remove pan from heat. Stir in butter, baking soda, and peanuts (mixture will foam). Stir until mixture is no longer bubbling and caramel is smooth, 1 minute.
3. Transfer to the prepared baking sheet and quickly spread with a lightly greased spatula. Let cool until firm, about 15 minutes. Break into pieces. The brittle can be stored in an airtight container at room temperature for up to 3 weeks.
Grease EVERYTHING! Before you begin this recipe, make sure that if you're not using a silicone baking mat (so perfect for candy!) that you spray your parchment paper with non-stick cooking spray, as well as the spatula you are going to use to spread the hot brittle. The hot caramel will be sticky, and having everything greased will allow you to work faster and get the brittle spread into a thin, even layer.
Prep the Ingredients - Have the peanuts, butter, and baking soda measured out and ready to go before you start the caramel. You'll need to add it all at once as soon as the caramel is ready and you don't want to have to pause to do any measuring.
Work QUICKLY! As soon as it's ready, you'll need to pour and spread the brittle onto the prepared pan very quickly. This is key to getting that thin "brittle" consistency; you don't want to be left with thick brittle!
You can use unsalted peanuts, salted peanuts, roasted peanuts, dry roasted peanuts, or raw peanuts in this recipe. Since we are adding the peanuts at the very end off the heat, roasted peanuts will not burn.