In a medium bowl, whisk together the flour, cocoa powder, cream of tartar, baking soda, cinnamon, and salt. Set aside.

In a large bowl using a hand mixer or stand mixer fitted with a paddle attachment, beat together the butter, granulated sugar, and brown sugar on medium-high speed until creamy and combined, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the eggs and vanilla extract. Beat on medium-high speed until combined. Scrape down the sides of the bowl with a spatula as needed.

Add the dry ingredients to the wet ingredients and beat on low speed until incorporated and combined. The dough will be thick.

Cover and chill the dough for at least 2 hours (or up to 3 days). If chilling for longer than a few hours, let the dough sit at room temperature for 20 minutes before scooping and rolling, as it will be quite firm.

Preheat the oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.

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

Scoop and roll cookie dough into balls, about 1.5 Tablespoons (35g) of cookie dough each. Roll the dough balls in the cinnamon-sugar topping. Arrange 2–3 inches apart on the lined baking sheets.

Bake the cookies until the edges are set but the centers look puffy, about 13–15 minutes. Allow cookies to cool on the baking sheet for 10 minutes and then transfer to a cooling rack to cool completely. Cookies will slightly deflate as they cool; that’s expected.

Store cookies in an airtight container at room temperature for up to 5 days.