01 - Preheat your oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together the flour, cocoa powder, cream of tartar, baking soda, and salt until well combined.
03 - In a large bowl, beat the butter and sugar together until light and fluffy, about 2-3 minutes.
04 - Add the egg and vanilla extract, mixing until fully incorporated into the butter mixture.
05 - Gradually add the dry ingredients to the wet mixture, mixing just until combined. Do not overmix.
06 - In a small bowl, mix together the 1/4 cup sugar and cinnamon for the coating.
07 - Scoop tablespoon-sized amounts of dough and roll into balls. Roll each ball in the cinnamon-sugar mixture to coat completely.
08 - Place the dough balls 2 inches apart on the prepared baking sheets.
09 - Bake for 9-11 minutes, or until the edges are set but the centers are still soft.
10 - Allow the cookies to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.