Preheat your oven to 350°F (180°C) and line a baking sheet with parchment paper.
In a large bowl, cream together the softened butter, granulated sugar, and light brown sugar until light and fluffy.
Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the flour, cocoa powder, cream of tartar, baking soda, and salt.
Gradually mix the dry ingredients into the wet ingredients until just combined.
In a small bowl, mix the remaining 1/4 cup granulated sugar and 2 teaspoons of ground cinnamon.
Roll the dough into 1-inch balls and then roll in the cinnamon-sugar mixture to coat.
Place the balls on the prepared baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes or until the edges are set and the centres are slightly puffed.
Let the cookies cool