Line a large baking sheet with parchment paper.

In a stand mixing bowl, beat the room temperature unsalted butter and dark brown sugar together for about three minutes until light and fluffy.

Add the large egg and vanilla extract to the mixture and mix until well combined.

In another bowl, combine the all-purpose flour, unsweetened dark cocoa powder, cream of tartar, baking soda, and salt.

Gradually add the dry ingredients to the butter-sugar mixture and mix until just combined.

Scoop the dough and roll it into balls using about two to three tablespoons of dough each.

Chill the dough balls in the freezer for three to four hours, or overnight for best results.

Preheat the oven to 350°F (180°C).

Mix granulated sugar and ground cinnamon in a small bowl.

Roll each chilled cookie dough ball in the sugar-cinnamon mixture until well coated.

Place the coated balls on the baking sheet, ensuring they are spaced two to three inches apart.

Bake for 10 to 13 minutes until the tops are crackled and the edges are set.

Allow cookies to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.