Fleur de Sel Toffee

Coat a rimmed baking sheet with cooking spray. Bring butter, sugar, water, and corn syrup to a boil in a large saucepan, whisking frequently until sugar dissolves and butter melts. Cook, undisturbed, until mixture registers 300 degrees on a candy thermometer, about 12 minutes.

Whisk toffee mixture, then immediately pour onto prepared sheet, tilting pan to spread over entire surface. Let stand for 30 seconds, then sprinkle with fleur de sel. Let cool. (Do not move the pan for first 30 minutes.) Break toffee into pieces.