In a large mixing bowl of a stand mixer or using an electric hand mixer, cream together butter, shortening and sugar until fluffy. Use a rubber spatula to scrape the bottom and sides of the bowl occasionally.
Add the eggs one at a time and combine.
Stir in the flour, cream of tartar, baking soda and salt until combined thoroughly. Scraping the bottom and sides of the bowl occasionally with a rubber spatula.
Cover the bowl with plastic wrap and refrigerate or transfer the dough into a large zip-style freezer bag and refrigerate for a minimum of 1-2 hours or overnight.
Preheat the oven to 400 degrees F.
Mix cinnamon and sugar in a small bowl using a spoon.
Shape the dough into 1 ¼-inch balls. Roll into the cinnamon sugar mixture. Place evenly on a half sheet pan lined with parchment paper, leaving a little bit of space in between each cookie. (24 cookies should fit on one half sheet pan.)
Bake for 8-10 minutes. Turn the sheet pans around half way through baking.
Remove from the oven and let cool on a baking rack.
Notes
As much as you want to add vanilla extract, don't. This cookie isn't meant to include it!We roll cookie dough into about 1 ¼-inches. That's a very small cookie. About two bites. You can make them larger if you prefer, but baking time will vary.