A layer of smooth chocolate and chopped hazelnuts disguise the top of this brittle as a candy bar.
Coat a 12-by-17-inch rimmed baking sheet withcooking spray, and line with parchment. Melt butterand sugar in a medium saucepan over medium-high heat. Add water and corn syrup. Cook until a candy thermometer reaches 300 degrees. Add 1/2 teaspoon salt. Stir to even out color. Pour mixture onto baking sheet without spreading. Let cool.
Spread chocolate over top, and sprinkle with hazelnuts. Refrigerate until firm, about 45 minutes. Break into pieces.