Preheat oven to 400°F, Line two baking sheets with parchment paper.
In a large bowl, cream together the softened butter, shortening and sugar until light and fluffy.
Add the eggs and vanilla extract, mixing until smooth.
In a separate bowl, whisk together the flour, cream of tartar, baking soda, and salt.
Add the dry ingredients to the wet ingredients and mix until a soft dough forms.
In a small bowl, stir together the cinnamon and sugar.
Roll the dough into 1½-tablespoon balls and coat each one generously in the cinnamon sugar.
Place on the prepared baking sheets, spacing them a few inches apart.
Bake for 9–10 minutes, or until the cookies puff up and the tops are slightly cracked.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.
