Sift the flour, cream of tartar, baking soda, and salt into a mixing bowl. Set aside. Combine the butter and shortening in the bowl of a standing electric mixer fitted with the paddle. Begin beating on low speed to blend together. Increase the speed to medium and beat for about 3 minutes, or until light and creamy. Add 1 1/2 cups of the sugar in a slow steady stream and continue to beat for 2 minutes. Reduce the speed to low and add the eggs, one at a time, scraping down the sides of the bowl with a rubber spatula after each addition. Add the vanilla and when blended, slowly beat in the dry mixture in two parts, beating until just incorporated. Remove the bowl from the mixer, scrape down the sides of the bowl, cover tightly with plastic wrap, and refrigerate for at least 8 hours or overnight. When ready to bake, preheat the oven to 350 degrees F. Combine the remaining 1/4 cup of sugar with the cinnamon in a large shallow bowl. Set aside. Line two baking sheets with nonstick silicone baking sheets or parchment paper. (Alternatively, use nonstick baking sheets or lightly butter conventional baking sheets.) Set aside. Remove the dough from the refrigerator and uncover. Roll the dough into 1 1/4-inch balls. Roll each ball in the cinnamon sugar to coat completely and place them about 21/2 inches apart on the prepared baking sheets. Place in the oven and bake for about 10 minutes, or until light brown. The cookies should be soft to the touch and leave a slight indentation in the center when touched. Do not overbake or they will be hard and dry. Remove from the oven and transfer the cookies to wire racks to cool. Store, airtight, at room temperature for up to a week.
Reprinted with permission from Milk & Cookies, Chronicle Books copyright (c) 2011