2 cups granulated sugar
1 cup water
2 tablespoons light corn syrup or agave nectar
2/3 cup toasted hazelnuts, coarsely chopped
1 1/2 teaspoons finely chopped fresh rosemary leaves
1/2 teaspoon fine salt
Line a baking sheet with parchment paper; set aside.
Combine the sugar, water, and corn syrup or agave in a medium heavy-bottomed saucepan over medium-high heat and stir until the sugar dissolves and the mixture comes to a boil, about 3 minutes. Continue to cook, swirling the pan occasionally but not stirring, until the mixture is an even, deep amber color (about 345°F on a candy thermometer), about 20 minutes more.
Immediately remove from the heat and stir in the hazelnuts, rosemary, and salt. Pour onto the baking sheet and use a rubber spatula to spread it into an even layer. Let cool completely until hardened and then use your hands to break it into pieces.