Preheat your oven to 350°F (177°C) and line two baking sheets with parchment paper.

In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg and vanilla extract to the creamed mixture, mixing until well combined.

In another bowl, whisk together the all-purpose flour, cocoa powder, cream of tartar, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

In a small bowl, mix together the granulated sugar and ground cinnamon for the coating.

Scoop out portions of dough and roll them into 1-inch balls, then roll in the cinnamon sugar mixture.

Place the dough balls on the baking sheets, spaced about 2 inches apart. Bake for 9-11 minutes.

Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.