1. Cream Butter & Sugars: In the bowl of your stand mixer (or a large bowl with a hand mixer), combine the softened butter with both brown and granulated sugars. Beat on medium speed for about 3 minutes until the mixture is beautifully light and fluffy. Don't forget to scrape down the sides of the bowl!
2. Add Wet Ingredients: Next, gently mix in the large egg, followed by the extra egg yolk, ensuring everything is well combined after each addition. Stir in the vanilla extract until it's just incorporated.
3. Combine Dry Ingredients: In a separate medium bowl, whisk together the flour, cocoa powder, baking soda, cream of tartar, and salt until evenly distributed.
4. Gradually Mix Dry into Wet: Add the dry ingredient blend to your butter and egg mixture in three stages. Mix on low speed until you see just a few small streaks of flour remaining – avoid overmixing to keep your cookies tender. Scrape the sides and bottom of the bowl with a spatula to ensure all ingredients are perfectly blended.
5. Initial Chill & Scoop: Pop the dough into the refrigerator for about 30 minutes. This helps it firm up a bit, making scooping much easier! Once slightly chilled, use a large cookie scoop to form 12 to 14 generous cookie balls and place them on a parchment-lined baking sheet.
6. The Essential Chill: For the best flavor and texture (think extra chewy and perfectly crinkly!), place these scooped dough balls back into the fridge. Chill them for a minimum of 3 hours, or even better, let them rest overnight.
7. Preheat & Prepare Coating: When you're ready to bake, preheat your oven to 350°F (175°C). Line a large baking sheet with parchment paper. In a shallow bowl, whisk together the remaining 1/4 cup granulated sugar and 1 teaspoon cinnamon for your signature coating.
8. Roll in Cinnamon Sugar: Take each thoroughly chilled cookie dough ball and generously roll it in the cinnamon-sugar mixture, making sure all sides are beautifully coated. This step creates those irresistible snickerdoodle cracks!
9. Bake to Perfection: Arrange the coated cookies on your prepared baking sheet, leaving about 2 to 3 inches between them as they will spread. Bake for 10 to 12 minutes, or until the tops of the cookies look wonderfully puffy and have started to crack, and their edges are just set.
10. Cool & Enjoy: Remove the cookies from the oven and let them cool on the baking sheet for 5 to 10 minutes before carefully transferring them to a wire rack to cool completely. They will settle and deflate slightly as they cool, revealing their rich, fudgy centers. Enjoy this delightful homemade chocolate treat!