In a medium bowl, mix together the flour, cocoa powder, cream of tartar, baking soda, cinnamon, chili powder, and cayenne if using. Set this aside for now.
In the bowl of stand mixer fitted with the paddle attachment, cream together the butter and ⅔ cup sugar on medium-high speed until light and fluffy (2-3 minutes). Then add the egg and vanilla and continue beating until creamy.
Add the dry ingredients to your mixer, fold them in until just combined. Wrap and transfer the dough to the fridge to chill for half an hour (or overnight). If chilling overnight, let the dough soften for 30 minutes first.
When you're ready to bake, preheat the oven to 350°F/180°C and line two baking sheets with parchment paper. Stir the ⅓ cup sugar and 2 tsp cinnamon together in a small bowl.
Form 1.25 inch balls of dough and roll them in the cinnamon sugar mixture. Place them onto the baking sheets, making sure to space them at least 2 inches apart. Bake for 10-11 minutes, or until the cookies are puffed up and edges are fully set. It's okay if the middle still looks a bit shiny.
Let the cookies cool for 5-10 minutes on the baking sheets before transferring to wire racks to cool completely. Optional: drizzle some melted chocolate over the top and sprinkle with a little more cinnamon sugar. Enjoy!