In KitchenAid or with electric mixer cream butter.
Add sugars and beat until light and fluffy.
Add eggs, one at a time, beating until combined and pausing after each addition to scrape down sides of bowl.
In separate, medium-sized bowl whisk together flour, cocoa powder, cream of tartar, baking soda, cinnamon and salt.
With mixer on low speed, gradually add flour mixture to butter mixture, pausing occasionally to scrape down sides of bowl.
Once all ingredients are incorporated, transfer bowl to refrigerator and chill 30 minutes.
In small bowl, whisk together sugar, ground cinnamon and cocoa powder for rolling.
Once dough has finished chilling, remove from refrigerator and roll into 1- 1 1/2" balls.
Roll in sugar/cinnamon/cocoa powder mixture until coated in mixture and place on a parchment paper-lined baking sheet, spacing about 2" apart (do not ever put cookie dough on a hot/too-warm pan, the cookies will spread too thin when baking if you do).
Place pan in refrigerator (or freezer!) and chill sugar/cinnamon/cocoa coated dough balls for another 30 minutes (this step is optional & can be skipped if you're impatient -- you can jump right to placing them in the oven. However, doing this step will give you nicer, more pronounced crackles in your cookies, if you're into that sort of thing)
While dough is chilling, preheat oven to 350F.
Once dough has finished chilling, bake on 350F for 10 minutes.
Remove from oven and allow to cool 5-10 minutes on cookie sheet before transferring to wire rack to cool completely.