1/2 cup water
2 cups granulated sugar
1/4 teaspoon cream of tartar
1 cup light corn syrup
2 teaspoons cinnamon
2 tablespoons unsalted butter
2 cups roasted, salted cashew nuts (if using unsalted cashew
nuts, add 1/8 teaspoon salt with the sugar)
1 teaspoon baking soda
Using vegetable oil, generously oil a sheet pan (preferably one with sides), at least 11 by 17 inches.
In a medium-size heavy saucepan, combine the water, sugar, cream of tartar and corn syrup and bring to a boil over medium heat. Using a candy thermometer to test it, boil the mixture until it reaches 350 degrees F, or is amber-colored. Remove from the heat and, working quickly, whisk in the cinnamon.
Whisk in the butter, cashews, and baking soda. Pour the mixture onto the oiled pan and spread it out a bit with a wooden spoon, to about 1/4 to 1/2-inch thickness. Don't spread it too thinly. Let harden, uncovered, in a cool place, 30 to 45 minutes. (To wash the saucepan, soak it overnight.)
Using your hands, break the brittle into pieces. Store in an airtight container.