01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - Whisk together flour, cocoa powder, cream of tartar, baking soda, and salt in a medium bowl.
03 - Beat butter and sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
04 - Mix in egg and vanilla extract until fully incorporated.
05 - Gradually add dry ingredients to wet mixture, mixing just until combined.
06 - Mix together 1/4 cup sugar and cinnamon in a small bowl.
07 - Scoop tablespoon-sized dough portions, roll into balls, and coat completely in cinnamon-sugar mixture.
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 rest on baking sheets for 5 minutes, then transfer to wire rack to cool.