Preheat your oven to 350°F (175°C).
In a medium bowl, whisk together the all-purpose flour, cream of tartar, baking soda, and salt. Set it aside.
In a large mixing bowl, combine the softened unsalted butter and vegetable shortening. Using your electric mixer, beat them together until light and fluffy, about 2-3 minutes.
Add the sugar to the butter mixture and beat until well combined. Then, add the eggs one at a time, mixing thoroughly after each addition.
Gradually add the dry ingredient mixture to the wet ingredients. Mix until just combined.
In a small bowl, combine the granulated sugar and ground cinnamon for the coating.
Using a cookie scoop or tablespoon, portion out the dough and roll each piece into a ball. Roll the balls in the cinnamon-sugar mixture until fully coated.
Place the coated dough balls on a parchment-lined baking sheet, spacing them about 2 inches apart. Bake for 8-10 minutes, or until the edges are lightly golden.
Remove from the oven and let cool on the baking sheet for a few minutes before transferring to a wire rack.