Generously butter a large baking sheet or line with a silicone mat; set aside.

Combine sugar, syrup, and water in a large heavy saucepan; cook over medium-high, stirring until sugar dissolves, then without stirring to 240°F on a candy thermometer.

Stir in cashews; cook, stirring frequently, to 300°F (hard crack) and light golden color.

Immediately remove from heat; quickly stir in butter, baking soda, vanilla, and salt. Mixture will foam.

Quickly pour hot brittle onto sheet; spread as thin as possible with a buttered spatula or wooden spoon.

Cool completely at room temperature about 1 hour; break into pieces and store airtight with wax paper between layers to prevent sticking.