Generously butter a large baking sheet or two smaller ones; measure all ingredients before cooking.

Combine sugar, corn syrup, and water in a large heavy-bottomed saucepan; cook over medium, stirring constantly, until sugar completely dissolves.

Stir in peanuts; cook 15-20 minutes, stirring frequently, until a candy thermometer reads 300°F (hard crack) and mixture turns golden amber.

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

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

Cool completely at room temperature about 1 hour; break hardened brittle with a spoon back or mallet. Store airtight up to 3 weeks.