Preheat your oven to 375 degrees Fahrenheit. Line a baking sheet with parchment paper and set aside. In a small bowl whisk together sugar and cinnamon. Set aside.
1/4 cup granulated sugar, 1/2 tablespoon cinnamon
In a medium bowl whisk together the all-purpose flour, cream of tartar, baking soda, and salt. Set aside.
2 1/2 cups all-purpose flour, 2 teaspoons cream of tartar, 1 teaspoon baking soda, 1/2 teaspoon table salt
In a large mixing bowl with a hand mixer or the bowl of a stand mixer with the paddle attachment, beat the butter, shortening, and sugar together until the mixture is light and fluffy, about three to six minutes.
8 tablespoons unsalted butter, 1/2 cup vegetable shortening, 1 1/2 cups granulated sugar
With the mixer on low, add in the eggs, one at a time, scraping down the sides as needed.
2 eggs
Add the flour mixture all at once. Set the mixer to very low and beat until it is just well enough combined that it won't fly everywhere. Turn up the mixer to high and beat until just combined, about 15 seconds. Give the batter a final stir with a spatula to make sure it is combined.
Optional: Let the cookies chill, covered, in the refrigerator for one hour. These will make for just slightly thicker and chewier cookies. My family loves them this way but also devours them when I don't chill the dough.
Using a 1 1/2 tablespoon cookie scoop, scoop the cookies, roll them into balls, and then roll them in the sugar and cinnamon mixture. Place on a parchment-lined cookie tray and bake (one tray at a time in the middle rack of the oven) 2 1/2 inches apart. Bake for 10 to 12 minutes rotating the tray halfway through. Remove the cookies when the edges just begin to turn golden brown. Let the cookies cool on the baking sheet for ten minutes before transferring to a wire rack.