01 - Preheat 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.
03 - In a large bowl, beat butter and sugar until light and fluffy, about 2-3 minutes.
04 - Add egg and vanilla extract, mixing until fully incorporated.
05 - Gradually add dry ingredients to wet mixture, mixing just until combined.
06 - In a small bowl, mix together 1/4 cup sugar and cinnamon for coating.
07 - Scoop tablespoon-sized amounts of dough and roll into balls. Roll each ball in cinnamon-sugar mixture to coat completely.
08 - Place dough balls 2 inches apart on prepared baking sheets.
09 - Bake for 9-11 minutes, until edges are set but centers remain soft.
10 - Let cookies cool on baking sheets for 5 minutes before transferring to wire rack to cool completely.