Preheat oven to 350 degrees Fahrenheit. Line baking sheets with parchment paper or silicone mats.

In the bowl of a stand mixer with the paddle attachment, cream the butter and sugars. Add eggs and vanilla. Scrape bowl. Mix well. 

In a separate large bowl, whisk together the flour, cocoa powder, cream of tartar, baking soda, baking powder, and salt. 

Add dry ingredients to the wet ingredients while mixing on low speed. Mix until just incorporated.

Scoop dough and round into balls (about 2 Tbsp each). Mix together the sugar and cinnamon in a bowl. Roll cookie dough balls in cinnamon-sugar and place on a prepared cookie sheet at least 2-inches apart. 

(Optional) CHILL dough on tray in the freezer for 10 minutes (or refrigerator for 30+ minutes) before baking for a softer and thicker cookie. 

OR ... Bake immediately for a flatter cookie that is crisp on the outside and tender in the center, with more cracks on top.

BAKE at 350˚F for 9-11 minutes, or until edges are set and the middle is still soft and slightly shiny. 

Immediately sprinkle extra cinnamon-sugar on top of the baked cookies while still hot, so it sticks. Let cookies rest on tray for 2 minutes before transferring to a rack to cool completely. Store leftovers in an airtight container.