Preheat the oven to 350 degrees F and line a rimmed baking sheet with parchment paper. Set aside.

To a medium bowl, add flour, cream of tartar, baking soda, salt, and cinnamon and whisk until combined. Set aside.

To a bowl of a stand mixer (this can also be done with a hand mixer), add the softened butter and sugars. Mix with a paddle attachment for 3-4 minutes on medium speed, or until the butter is light and fluffy and no clumps. Be sure to scrape down the sides of the bowl. Add in the eggs and mix for another 2 minutes on medium speed, or until the mixture is a pale yellow color and fluffy.

Add in the sourdough discard and vanilla extract and mix on low until everything is combined.

While mixing on low speed, gradually add in the dry ingredients and stop once the flour just has a few flour streaks. With a rubber spatula, scrape the sides of the bowl and mix the rest of the remaining flour into the dough by hand.

With a cookie scoop, or spoon, scoop 35g of dough per cookie or about 2 Tablespoons. This dough is very soft, but shouldn’t be sticky. Once rolled, it should form a smooth dough ball.

In a medium bowl, whisk together the cinnamon and sugar. Roll the cookie dough into a smooth, round dough ball, then roll in the cinnamon-sugar mixture until completely coated.

Place the cookies on the prepared baking sheet a few inches apart for 9-11 minutes. The cookies are ready when the edges are just beginning to set, the tops will look mostly matte with a bit of shine in the middle.

Immediately after you remove the cookies from the oven, use the backs of two spoons to push the cookies towards the center.

After shaping, leave the cookies on the baking sheet for 5 minutes before moving them to a wire rack to cool.

Repeat with the remaining cookie dough to make about 30 cookies total.