Preheat oven to 375°F (190°C). Line two large cookie sheets with parchment paper or silicone baking mats. Set aside.

Make the cookies: Whisk the flour, cream of tartar, baking soda, cinnamon, and salt together in a medium bowl.

In a large bowl using a hand mixer or stand mixer fitted with a paddle attachment, beat the butter and granulated sugar together on high speed until smooth and creamy, about 2 minutes. Add the egg, egg yolk, and vanilla extract. Beat on medium-high speed until combined. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. With the mixer running on low speed, slowly add the dry ingredients to the wet ingredients in 3 different parts. The dough will be thick.

Make the topping: Combine the granulated sugar and cinnamon together in a small bowl.

Roll cookie dough into balls, about 1.5 Tablespoons (35g) of cookie dough each. Roll the dough balls in cinnamon-sugar topping. Arrange 3 inches apart on the baking sheets.

Bake cookies for 10 minutes. The cookies will be very puffy and soft. When they are still very warm, lightly press down on them with the back of a spoon or fork to help flatten them out. Allow cookies to cool on the baking sheet for 10 minutes and transfer to a wire rack to cool completely.

Cookies remain soft & fresh for 7 days in an airtight container at room temperature.