Line a large rimmed baking sheet with a silicone mat or piece of parchment paper. Set aside.
In a large stock pot fitted with a candy thermometer, bring the water, granulated sugar, light corn syrup, and unsalted butter to a boil. This should take anywhere from 12 to 16 minutes. DO NOT STIR OR DISTURB THE COOKING SUGARS UNTIL YOU START TO SEE THEM GETTING LIGHTLY GOLDEN IN COLOR. You want your brittle mixture to reach 300°F (hard crack stage) on your candy thermometer. Turn off the heat to the pot immediately. If you have an electric stove, remove the pot from the burner so that your sugar mixture does not keep cooking.
Working very quickly, stir in the baking soda, vanilla extract, and cashews. Be very careful as the baking soda will cause the sugar mixture to bubble up rapidly.
Immediately pour your hot brittle mixture onto your prepared baking sheet and quickly spread out the cashew brittle into as even a layer as possible (an approximate 9×12-inch rectangle). You will need to work quickly as the brittle will start to harden almost immediately.
Allow your cashew brittle to cool on the counter completely before breaking it into pieces.