Prepare Oven and Baking Surfaces: Preheat your oven to 375°F (190°C). Line two large baking sheets with silicone baking mats or parchment paper to prevent sticking and promote even baking. Set these aside while you prepare the dough.
Combine Dry Ingredients: In a medium-sized bowl, whisk together the flour, cream of tartar, baking soda, cinnamon, and salt until fully blended. This mixture will give your cookies their signature tang and tender crumb.
Cream Butter and Sugar: Using a stand mixer fitted with the paddle attachment or a handheld electric mixer, beat the softened butter and granulated sugar on high speed for about 2 minutes. Continue until the mixture becomes light, creamy, and smooth — this is crucial for a soft texture.
Add Eggs and Vanilla: Add the whole egg, extra egg yolk, and vanilla extract to the butter-sugar mixture. Mix on medium-high speed until the wet ingredients are well combined and slightly fluffy. Scrape down the bowl as needed to ensure even mixing.
Incorporate Dry Ingredients Gradually: Switch the mixer to low speed and add the flour mixture in three increments, mixing just until incorporated after each addition. The dough will be thick and slightly sticky, perfect for forming cookie balls.
Prepare Cinnamon Sugar Coating: In a small bowl, blend together the sugar and cinnamon for the topping. This sweet, spicy coating will give your snickerdoodles their classic finish.
Shape and Coat Dough Balls: Using a medium cookie scoop (about 1.5 tablespoons), portion the dough into even balls. Roll each ball thoroughly in the cinnamon sugar mixture, ensuring an even coating on all sides. Place them on the prepared baking sheets about 3 inches apart to allow for spreading.
Bake Cookies: Bake the cookies in the preheated oven for exactly 10 minutes. The cookies will puff up and look soft and tender when done — avoid overbaking to keep them soft.
Lightly Flatten Cookies While Warm: As soon as the cookies come out of the oven and are still warm, gently press the tops with the back of a spoon or fork to slightly flatten them. This helps achieve the classic snickerdoodle shape and texture.
Cool Completely: Allow the cookies to cool on the baking sheets for about 10 minutes before transferring them to a cooling rack. Cooling on the rack prevents sogginess and maintains their perfect soft texture.