In a large bowl, beat together the butter, granulated sugar, and brown sugar until light and fluffy.
Add the eggs, one at a time, followed by vanilla.
Add the flour, cocoa powder, cream of tartar, baking soda, and salt. Mix until just combined with no streaks of flour remaining.
Cover the dough and refrigerate for about an hour until well chilled.
Preheat oven to 375˚F (190˚C). Line two baking sheets with parchment or lightly grease.
In a wide bowl, mix together sugar and cinnamon to make the topping.
Use a small scoop or spoon to take out heaping tablespoons of the chilled dough and form into balls.
Roll each ball thoroughly in the cinnamon sugar and place on the prepared baking sheets 2 inches (5 centimeters) apart.
Bake in preheated oven for 10-13 minutes, until just set and lightly browned on bottom.
Allow to cool on the baking sheets for 10 minutes before removing to wire racks to cool to room temperature.
Store in an airtight container at room temperature for up to three days.