Start by lining a baking sheet with parchment paper. This will prevent the brittle from sticking once it cools.
In a medium-sized saucepan, combine the granulated sugar, water, and light corn syrup. Stir gently until the sugar is dissolved.
Place the saucepan over medium heat and attach your candy thermometer to the side. Allow the mixture to boil without stirring until it reaches 300°F (hard crack stage).
Once the temperature reaches 300°F, remove the saucepan from the heat. Quickly stir in the unsalted butter, baking soda, roasted & salted cashews, and vanilla extract.
Immediately pour the cashew mixture onto the prepared baking sheet. Use a heat-resistant spatula to spread it out evenly.
Allow the cashew brittle to cool completely at room temperature. Once it’s set, break it into pieces and enjoy!