Directions

1. Preheat the oven to 400 degrees, with one rack in top third and one rack in bottom third of oven. Line baking sheets with Silpat baking mats or parchment paper; set aside.
2. Sift together flour, cream of tartar, baking soda, and salt; set aside. In the bowl of an electric mixer fitted with a paddle attachment, combine butter, shortening, and 1 1/2 cups sugar. Beat on medium speed until light and fluffy, about 2 minutes. Scrape down sides of bowl. Add eggs, and beat to combine. Add dry ingredients, and beat to combine.
3. In a small bowl, combine remaining 1/4 cup sugar and the ground cinnamon. Use a small (1 1/4-ounce) ice-cream scoop to form balls of the dough, and roll in cinnamon sugar. Place about 2 inches apart on the prepared baking sheets. Bake until the cookies are set in center and begin to crack (they will not brown), about 10 minutes, rotating the baking sheets after 5 minutes. Transfer the sheets to a wire rack to cool about 5 minutes before transferring the cookies to the rack. Store in an airtight container up to 1 week.