Using butter, generously grease a sheet pan (preferably one with sides), at least 11 x 17 inches (or use a SILPAT®).
Mix together cinnamon and baking soda; set aside.
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°F
Remove from the heat and, working quickly, whisk in the cinnamon and baking soda mixture, butter and cashews.
Pour the mixture onto the oiled pan and spread it out a bit with buttered metal spoon or a wooden spoon to about 1/4- to 1/2-inch thickness. Don't spread it too thin. Let cool and harden, uncovered, 30–45 minutes. Using your hands, break the brittle into pieces. Store in an airtight container.
Tip: To wash the saucepan, fill it with water and let it soak overnight.