Preheat the oven to 350 degrees F. Line a large baking sheet with a Silpat baking mat or parchment paper. Set aside.

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

In the bowl of a stand mixer, beat the butter until smooth, about 30 seconds. Add the brown sugar and granulated sugar and beat until light and fluffy, about 2 minutes. Scrape down the sides and add the eggs and vanilla. Mix until combined.

Add the dry ingredients and turn the mixer on low. Mix until just combined.

In a small bowl, combine the sugar and cinnamon. Stir until well combined.

Form the dough into balls, about 2 tablespoons per cookie. Roll the cookie dough balls in the cinnamon sugar mixture. Place on prepared baking sheet, about 2 inches apart.

Bake cookies for 8-10 minutes or until cookies are set around the edges but still soft in the center. Don't overbake.

Let the cookies cool on the baking sheet for about 5 minutes. Transfer to a cooling rack to cool completely.