Preparation

• Preheat oven to 375°F (190°C). Line two large cookie sheets with parchment paper or silicone baking mats.
• Combine the granulated sugar and cinnamon together in a small bowl.
• 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.
• 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.