Have ready a 10x15-inch rimmed baking sheet. If the sheet is not nonstick, lightly coat it with oil.
Combine the sugar, corn syrup, butter, and 2/3 cup water in a heavy-duty 4-quart saucepan. Stir until all of the sugar is wet. Clip a candy thermometer onto the pan and cook the mixture over medium-high heat without stirring (it will be foamy) until it registers 310°F and is tan around the edges, 12 to 16 minutes.
Remove from the heat and quickly remove the thermometer. Whisk in the baking soda (it will be foamy), and then the salt. Whisk in most of the seeds or nuts or all of the spice of your choice. Pour the mixture onto the baking sheet and tilt the sheet to spread it evenly. Sprinkle any remaining nuts or seeds on top. Cool completely on the baking sheet on a rack at room temperature, about 1-1/2 hours.
Invert the brittle onto a large cutting board. Use a meat mallet, rolling pin, or your hands to break it up into medium-to-small pieces. The brittle can be stored in an airtight container at room temperature for up to 1 month.
nutrition information (per serving):
2 Tbs., Calories
2, Fat Calories
20, Saturated Fat
0, Monounsaturated Fat
6, Polyunsaturated Fat
Photo: Scott Phillips
Several reviewers below noted that their brittle came out too salty. The test kitchen responds: The Fine Cooking test kitchen mainly relies on Diamond Crystal brand kosher salt for testing, though not all kosher salt crystals are equal. Morton’s Kosher, for example, has a denser flake which, when used in the same volume as the Diamond, is much more powerful. If you’re measuring by volume, 1 Tbs. Morton’s = 1-1/2 Tbs. Diamond. In this recipe, you can be sure you have the right amount by weighing your salt.
Also keep in mind that table salt is even more dense, and using 1-1/2 Tbs. here will render the toffee inedible. Likewise, if you substitute salted butter or salted nuts for unsalted, that will only add to the saltiness.