Recipe very slightly adapted from this recipe on Inspired Taste. Makes 20-22 3-inch cookies.
2 1/2 cups all-purpose unbleached flour
1/2 cup unsweetened cocoa powder
2 tsp cream of tartar
1 tsp baking soda
1 tsp cinnamon
1/2 teaspoon kosher salt
16 Tbsp (2 sticks) unsalted butter, at room temperature
1-1/2 cups granulated sugar
1/2 tsp pure vanilla extract
2 large eggs, at room temperature
For the coating:
1/4 cup granulated sugar
2 tsp ground cinnamon
Preheat oven to 400°F. Line two rimmed baking sheets with Silpats (silicone mats) or parchment paper, and set aside.
In a large mixing bowl, whisk together the flour, cocoa powder, cream of tartar, baking soda, cinnamon and salt.
In the bowl of a Kitchenaid-type stand mixer, cream together the butter, sugar and vanilla on medium speed for 2 minutes, until the mixture is well combined and has turned a lighter shade of yellow. Slowly add in the flour mixture, a bit at a time, and stop mixing when all of the dry ingredients have been incorporated.
Cover the mixer bowl with plastic wrap, and refrigerate for 20-30 minutes.
In a small mixing bowl, stir together the ingredients for the coating.
Use a spoon or a disher (an ice cream scoop with a release) to form balls of cookie dough. Roll each one in your hand until smooth, then gently roll it in the cinnamon-sugar mixture. Place on the baking sheet 3 inches apart (you'll get 12 cookies on a baking sheet).
Bake for 10 minutes, until the top crinkles. Remove the pans from the oven (if you're using two racks, move the baking sheet on the lower rack to the upper, and bake for an extra 1-2 minutes), and set on the stovetop to cool for 5 minutes.
Then, transfer the cookies to a cooling rack and let them cool until crispy on the outside.
Store in a container with a tight-fitting lid, for a couple of days.