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.
