Preheat the oven to 400F. Line a baking sheet with parchment and set aside.
In a bowl, mix together the flour, cream of tartar, baking soda, and salt. Set aside. In a stand mixer, cream together the butter, shortening, and sugar on medium speed for 3 minutes.
Add the eggs one at a time on medium speed. Scrape down the sides of the bowl with a spatula to make sure everything is well incorporated.
Add the dry ingredients in two to three additions on low speed.
In a shallow bowl, mix together the 3 tbsp of sugar and cinnamon together.
Take one tablespoon of the cookie dough and roll the balls into the cinnamon sugar mixture. Place the balls on the prepared baking sheet and bake for 10 minutes. Let cool on a cooling rack before serving.