In a medium bowl, whisk together flour, cocoa, cream of tartar, baking soda, and salt.
In a large bowl using a hand mixer, or in the bowl of a standing mixer, cream the butter and sugars together at medium/high speed. Add eggs and vanilla and mix until thoroughly combined.
Add dry ingredients to wet ingredients and mix well.
Chill for an hour or up until 2 days. When ready to bake, preheat oven to 400 F.
Using about 1 tablespoon of dough, roll into balls then roll in the cinnamon-sugar mixture.
Place about 2″ apart on cookie sheet. Cookies will spread.
Bake for 8-10 minutes. Makes about 3 dozen cookies.