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, 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.
Courtesy Gale Gand, "Butter Sugar Flour Eggs" by Gale Gand, Rick Tramonto, Julia Moskin, Clarkson N. Potter Publishers, 1999