1. Preheat oven to 350°F (177°C). Line baking sheets with parchment paper.

2. In a large bowl, cream together butter, granulated sugar, and brown sugar until light and fluffy.

3. Add the egg and vanilla extract and mix until well combined.

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

5. Gradually add dry ingredients to the wet ingredients, mixing until just combined.

6. In a small bowl, mix 1/4 cup granulated sugar and 1 1/2 teaspoons cinnamon.

7. Scoop dough into tablespoon-sized balls and roll each ball in the cinnamon sugar mixture.

8. Place on prepared baking sheet, spaced 2 inches apart.

9. Bake for 9–11 minutes, until edges are set but centers are soft.

10. Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.