Heat oven to 375°F and line 3 baking sheets with parchment.
In a large bowl, whisk together flour, cream of tartar, baking soda and salt; set aside.
Use a stand mixer fitted with the paddle attachment to cream together shortening and butter for 30 seconds. Add sugar and beat an additional 3 minutes or until lighter in color and fluffy
Beat in eggs, one at a time, just until incorporated.
Add the flour mixture (all at once) to the creamed butter mixture and mix on low just until no flour is visible. Do not over work the dough or the cookies will be tough.
Set up a “work station” near the baking sheets with the bowl of cinnamon/sugar and a bowl of iced water. Set a folded paper towel next to the water bowl.
Gently roll 2 tablespoons dough into a ball, dip the dough into the iced water and briefly place it on the paper towel (to absorb excess water). Roll the cookie dough ball in the cinnamon sugar and set on a baking sheet. Repeat with all the dough; spacing the balls 2-inches apart. There should be 8 cookies per sheet.
Bake the cookies, on the middle rack, at 375°F, for 10-12 minutes (rotating after 6 minutes). The cookies are done when the edges are
just set and barely browning, and the centers are soft, cracked, and will look slightly undercooked.
Cool the cookies 5-8 minutes on the baking sheet then transfer to a cooling rack.
Keep the cookies in an airtight container up to 5 days or freeze for later use.