You can also make this caramel brittle with other nuts, such as slivered almonds or pistachios.
Source: Martha Stewart Living, November 1998
Yield Makes one 11-by-17-inch sheet
Vegetable oil, for pan and knife
4 cups sugar
1/4 teaspoon apple-cider vinegar
5 1/2 cups hazelnuts
Oil an 11-by-17-inch baking pan. Stir sugar, vinegar, and 1 cup water in a medium saucepan over medium heat until sugar dissolves. Cook without stirring until dark amber, 18 to 25 minutes. Stir in nuts.
Pour mixture onto pan. Let set until firm but still soft enough to cut. Unmold onto a cutting board. Working fast, use an oiled chef's knife to cut into six rectangles.
When making caramel for brittle, always use a heavy-bottomed pan to evenly distribute heat. When caramelizing sugar and water, reduce the heat after the syrup boils, and gently swirl the pan to check the color. To prevent crystallization, brush down the sides of the pan with a pastry brush dipped in water; covering the pan for a few minutes also prevents crystallization, for any condensation will become trapped and run down the sides. To stop the cooking process, dip the pan in a bowl of ice water for a few seconds. For easier cleanup, place utensils in the pan, fill with water, and bring to a boil until sugar has dissolved.