Preheat the oven to 375°F and line two large baking sheets with parchment paper or silicone baking mats. Set them aside for later.

In a medium bowl, whisk together the flour, cream of tartar, baking soda, ground cinnamon, and salt until everything is evenly combined.

In a large mixing bowl, beat the butter and granulated sugar on high speed using a hand mixer or a stand mixer fitted with the paddle attachment. Continue until the mixture is light and fluffy, about 2 minutes. Add the egg, egg yolk, and vanilla extract, then mix on medium-high speed until fully incorporated, scraping down the sides and bottom of the bowl with a rubber spatula as needed.

Reduce the mixer speed to low and gradually add the dry ingredients to the wet mixture. Mix until just combined, being careful not to overmix.

Roll the dough into small balls, using about 2 teaspoons of dough for each one. Roll each ball in the cinnamon sugar mixture to evenly coat. If you want extra flavor, sprinkle a little more cinnamon sugar on top.

Arrange the dough balls on the prepared baking sheets, leaving a couple of inches between each one. Bake for 5-7 minutes, or until the cookies look puffy and soft.

While the cookies are still warm, gently press down on the tops with the back of a spoon to flatten them slightly. Let them cool on the baking sheet for about 5 minutes, then toss in sugar cinnamon again before transferring to a wire rack to cool completely.