In a small bowl, add the flour, cocoa powder, cream of tartar, baking soda, and salt then whisk until well mixed.
Add the butter, brown sugar, and granulated sugar to a stand mixer bowl and cream everything together on medium.
Once the butter and sugars are fluffy and have lightened in color, turn the mixer speed down to low and add one egg. Add the second egg after the first has mixed in then add the vanilla.
With the mixer still on low, slowly add the previously combined dry ingredients. Stir until the flour mix has barely been blended in, turn off the mixer, then mix by hand with a spatula a final few times.
Let the dough rest on the counter or in the refrigerator for 30 minutes. About 15 minutes into resting, begin preheating the oven to 400° F (204° C) and line a baking sheet with parchment paper or a silicone baking mat. In a small bowl, combine the remaining 2 tablespoons of sugar with the 2 teaspoons of cinnamon.
Scoop about a 1 tablespoon sized hunk of cookie dough, roll it gently into a ball, then roll it around in the cinnamon and sugar, covering it completely.
Set the balls of dough on the baking sheet about 3 inches apart to account for spread as they bake.
Bake for about 8 minutes. The edges will be slightly puffy and dry while the rest of the cookie will be crackled. Cool the cookies on the baking sheet for about 10 minutes then remove them to a rack to cool completely.