Line a rimmed baking sheet with parchment paper or a silicone mat and spray it lightly with nonstick cooking spray.
In a large heavy-bottomed saucepan, combine the sugar, corn syrup, and water. Stir the mixture over medium heat until the sugar has dissolved completely.
Bring the syrup to a boil and cook until it reaches the hard crack stage (300-310 °F (149-154 °C)- about 8-10 minutes. You don’t want to stir the syrup while it’s cooking but you can carefully swirl the pot as needed. Make sure to watch the syrup closely as it can burn very quickly!
Once the syrup reaches the hard crack stage, remove the saucepan from heat. Quickly add the butter, vanilla extract, and baking soda. Stir to combine.
Next add the cashews and stir to coat the nuts evenly. The mixture will cool very fast so make sure to work quickly.
Pour the cashew mixture onto the prepared baking sheet and use a spatula to spread it out into an even layer– about ¼ inch thick. You can also place a piece of greased parchment paper on top of the candy and use the parchment to press it into an even layer.
Allow the cashew brittle to cool at room temperature until it hardens– about 30 minutes.
When the brittle is set, break it into your desired size pieces and transfer to an airtight container. Store at room temperature for up to one week.
Notes
Roasted, salted nuts are a must for this sweet and salty candy! Feel free to use other types of salted nuts, like peanuts, pecans, or almonds in this recipe.
The toffee may bubble up when you add the baking soda. Don't worry, just stir until the mixture calms down a bit. The baking soda is important in this recipe, as it helps to soften the candy so that it's easy to bite into.
Avoid trying to leave out or substitute anything for the corn syrup. This liquid sweetener is the best option for making cashew brittle that doesn't turn grainy and hard.
Use a candy thermometer. Until you are a pro at candy making, you'll want to be sure that the sryup is at the right "stage". In this case, you're looking for a "hard crack stage", or a temperature of 300-310 °F (149-154 °C). You can use a manual thermometer or a digital thermometer made for candy making.
You should be able to easily break the brittle apart with your hands, but for more uniform pieces, you can try using a long knife to encourage the brittle to break in straighter lines.
Store homemade cashew brittle in an airtight container at room temperature for up to a week.