Generously butter two large rimmed baking sheets. Set aside.
Combine sugar, corn syrup, and water in 3-qt saucepan. Heat over medium heat, stirring occasionally, until mixture comes to a full boil.
Add butter. Continue cooking 15-25 minutes until candy thermometer registers 280 degrees F. Continue stirring occasionally.
Add peanuts. Stir continuously for 10-12 minutes until candy thermometer registers 305 degrees F. Remove from heat and stir in baking soda.
Pour evenly into prepared pans. Do not spread.
Cool completely then break into pieces.
Notes
Test your candy thermometer before you start making this recipe. Bring a pot of water to a boil. After it's been boiling for 1 minute, insert your candy thermometer into the water and wait until it reaches temperature. It should read 212 degrees F.
Once your peanut brittle mixture reaches 305 degrees F, it's important to work quickly to stir in the baking soda.
Don't over-stir the mixture or all the bubbles will burst and you won't get that light and airy texture.
Grease the pan with butter for extra flavor. It'll keep the candy from sticking to the pan, too.
Make sure you pour the hot candy into a rimmed baking sheet so that it won't spill off the edges.
Try to pour the hot candy in a wide circle in the pan to distribute it evenly. You don't want to pour the candy all in one place or it might be too thick and hard to bite into.
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