Line a large baking sheet with parchment paper and set it aside.
In a stand mixing bowl, cream the butter and sugar together for 3 minutes until light and fluffy.
Then mix in the egg and vanilla extract.
Mix in the flour, cocoa powder, salt, cream of tartar, and baking soda until JUST combined. Scoop the dough into 2 tablespoon-sized balls or 3 tablespoon-sized balls (for larger thicker cookies) onto the prepared baking sheet. Chill the dough in the freezer for 3-4 hours, overnight is best!
Preheat the oven to 350 F/180 C.
Mix the sugar and cinnamon together. Roll each frozen cookie dough ball in the mixture until well coated, then place them on a baking sheet spaced 2-3 inches apart. You will need to bake them in 2 batches to not crowd the pan. Keep the rest of the dough balls in the freezer until ready to bake!
Bake the cookies for 10-12 minutes for smaller cookies, or 12-13 minutes for larger cookies, until the tops are crackled and the edges are set. Let the tray cool on a wire rack before removing them. Enjoy!
Store leftover cookies in an airtight container at room temperature for 2-3 days. You can freeze the cookie dough in an airtight container or freezer bag for up to 3 months. I recommend waiting to roll them in cinnamon sugar until right before baking, otherwise the sugar will start to melt in the freezer.