Preheat the oven to 350 F. In a medium bowl, whisk together flour, cocoa powder, salt, baking soda, and cream of tartar. In a large bowl, combine butter and sugar until fluffy.
Add in the egg and vanilla extract, mix until smooth. Add in the flour mixture, and mix until it is fully combined. In a small bowl, combine sugar and cinnamon.
Press the cookie dough into a greased 8-inch square baking dish. Sprinkle the dough evenly with the cinnamon and sugar mixture. Bake for 20-25 minutes.