Line a rimmed baking sheet with parchment paper and butter both sides of a rubber or silicone spatula which you'll use to spread brittle after pouring it on the baking sheet.
Combine sugar, water, butter and corn syrup in a medium saucepan and place over medium-high heat. Cook and stir until the mixture comes to a boil. Cook, stirring often, until caramel temperature reaches 300°F on a candy thermometer (hard crack stage). Remove from heat and stir in baking soda (mixture will get foamy and increase in volume). Stir in nuts and scrape onto prepared baking sheet. Use the buttered spatula to help you spread the brittle in a single layer that is as even as possible. Sprinkle lightly with flaky sea salt.
Let cool completely and break into one or two-bite pieces.
Notes
Make sure that your saucepan is large enough to accommodate the sauce when it bubbles up with the addition of baking soda and large enough to allow you to stir in the cashews.
You'll need to work quickly to spread the brittle on the baking sheet. With the extra cashews, it begins to harden almost immediately.
Store cashew brittle in an airtight container at room temperature for up to 8 weeks. Do not refrigerate or your brittle will soften and become chewy.