1. Preheat the oven to 350°F and line two baking sheets with parchment paper.

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

3. In a large bowl, beat the softened butter, granulated sugar, and brown sugar until light and fluffy.

4. Add the egg and vanilla extract, then mix until smooth and fully combined.

5. Gradually add the dry ingredients to the wet ingredients and mix just until a soft dough forms.

6. In a small bowl, stir together the coating sugar and cinnamon.

7. Scoop the dough into 1 1/2 tablespoon portions and roll each portion into a ball.

8. Roll each dough ball in the cinnamon-sugar coating until fully covered.

9. Place the dough balls on the prepared baking sheets about 2 inches apart.

10. Bake for 9 to 11 minutes, until the edges are set and the centers are still slightly soft.

11. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.