Preheat the oven to 400 degrees with the racks set to the top middle and bottom middle positions. Line two baking sheets with parchment paper and set aside.

In a large bowl, whisk the cream of tartar, baking soda, and salt.

In the bowl of your stand mixer, fitted with the paddle attachment, cream the butter, shortening and 1 1/2 cups sugar until creamy. Scrape down the sides, and mix in the eggs one at a time.

Add the flour to the butter mix and beat for about 30 seconds to combine.

In a small bowl, combine the remaining 3 tbs sugar with the cinnamon. Roll the dough into 1 1/2" balls and toss into the cinnamon sugar mix to coat. Place each on the baking sheets about 2 inches apart.

Bake the cookies about 9 to 11 minutes, rotating racks halfway through cook time.

Allow the cookies to cool 4 minutes on baking sheets before transferring to a wire rack to cool completely.