In the bowl of a stand mixer fitted with the paddle attachment, mix together the butter and sugars on medium speed until light and fluffy, about 3 minutes. Scrape down the sides of the bowl. Add the egg, followed by the egg yolk. Mix well between each addition. Add the vanilla. Mix on low speed to combine. Scrape down the sides of the bowl. In a separate bowl, whisk together the flour, cocoa powder, baking soda, cream of tartar, and salt. Add the dry ingredients to the butter and egg mixture in 3 additions. Pro-tip: Cover the mixer with a dishtowel or wrap in plastic wrap to keep the cocoa powder from flying out of the mixing bowl. Mix until just a few small streaks of flour remain. Do not overmix. Scrape down the sides and bottom of the bowl with a large spatula to be sure all of the ingredients are incorporated. Place the dough in the refrigerator to chill for about 30 minutes. This helps to firm up the dough so the scoops will hold their shape. Use a large cookie scoop to divide the dough into 12 to 14 large cookies. Place on a parchment-lined baking sheet. Chill for 3 hours or overnight. Once the dough is chilled, whisk together the remaining sugar and cinnamon in a shallow bowl for rolling the cookies. Preheat the oven to 350°F. Line a large baking sheet with parchment paper. Dunk each chilled cookie dough ball in the bowl of cinnamon sugar and roll to cover generously on all sides. Place the cookies on the prepared baking sheet about 2 to 3 inches apart. The cookies will spread in the oven. Bake for 10 to 12 minutes or until the tops of the cookies are puffy and beginning to crack. The edges of the cookies should be set. Remove the cookies from the oven and cool on the baking sheet for 5 to 10 minutes before carefully transferring to a wire rack to cool completely. The cookies will deflate slightly as they cool.