Line a 13 x 18-inch baking sheet (half-sheet pan) with parchment paper and set aside. (If you prefer very thin toffee, you may need a larger pan or distribute the cashew crunch into two pans.)
Add the water, light corn syrup, and sugar to a large heavy saucepan. (The mixture will bubble up, so you will need to use at least a 2 1/2-quart sized pan.) Bring the mixture to a boil over medium-high heat, stirring occasionally with a wooden spoon.
1/2 cup Karo light corn syrup, 2 cup sugar, 1 cup salted butter at room temperature (2 sticks), 1/2 cup water
Add the butter and cook the mixture, stirring occasionally, over medium high heat, until it reaches the soft-crack stage (280℉) on a candy thermometer.
Once the temperature reaches approximately 280℉, slowly add the salted cashews to the mixture, and keep stirring continuously with a wooden spoon until it reaches the hard-crack stage (approximately 295℉).
2 1/2 cups salted cashews
Remove the pan from the heat and immediately sprinkle the baking soda over the mixture, stir quickly to incorporate.
1 teaspoon baking soda
Immediately pour the mixture onto the prepared baking sheet, spreading it evenly to the edges.
If you want a salty toffee, feel free to sprinkle a bit of coarse salt over the top after spreading it in the pan. Allow the brittle time to cool and harden at room temperature.
Once completely cooled, break the cashew crunch into bite-sized pieces. Store in an airtight container at room temperature.