Generously butter a large baking sheet or line with a silicone baking mat. Set aside.
In a large heavy saucepan, combine sugar, corn syrup, and water. Cook over medium-high heat, stirring until sugar dissolves. Continue cooking without stirring until the mixture reaches 240°F on a candy thermometer.
Stir in cashews and continue cooking, stirring frequently, until the mixture reaches 300°F (hard crack stage) and turns a light golden color.
Remove from heat immediately and quickly stir in butter, baking soda, vanilla, and salt. The mixture will foam up – this is normal!
Working quickly, pour the hot brittle onto the prepared baking sheet and spread as thin as possible using a buttered spatula or wooden spoon.
Allow to cool completely at room temperature, about 1 hour, then break into pieces. Store in an airtight container with wax paper between layers to keep pieces from sticking together.