Prepare a 9" x 13" sheet pan by lining it with a silicone baking mat or parchment paper, or lightly grease with butter.
In a heavy-bottomed saucepan, combine granulated sugar, water, butter, and corn syrup.
Heat over medium-high, stirring occasionally, until the mixture begins to boil.
Wipe the inside of the pan with a clean brush dipped in cold water to remove any sugar crystals.
Continue cooking until the caramel reaches 300°F, about 8-12 minutes.
Remove from heat, add baking soda, and stir briefly (careful, it may sputter).
Add the nuts and stir quickly to coat them with the caramel.
Pour the mixture onto a heat-resistant surface (like Silpat) and smooth with a spatula.
Let the brittle cool completely (about 40 minutes) and break it into pieces. Enjoy!