- Preheat oven to 375F.
- In a bowl, whisk together flour, cream of tartar, baking soda and salt until well blended. Set aside.
- In a large mixing bowl or the bowl of a stand mixer, beat butter and sugar on medium-high speed until light and fluffy, 1 to 2 minutes.
- Add molasses and beat until combined, scraping down the sides of the bowl as needed.
- Add egg and vanilla and mix until smooth.
- Add dry ingredients and mix on low speed until incorporated and no dry flour remains.
- In a small dish, mix raw sugar together with cocoa powder and cinnamon until well blended.
- Drop dough by the tablespoonful into sugar mixture, rolling until completely coated. (Dough will be sticky, but the sugar mixture should keep it from sticking to your hands.).
- Arrange on non-stick or parchment-lined baking sheets, leaving 2 inches of space between cookies. Repeat with remaining dough.
- Bake for 9 to 10 minutes or until cookies are just set on top and bottoms are lightly golden brown.
- Let cool for 5 minutes on baking sheets and transfer to a wire rack to cool completely.
Cookies will keep, stored in an airtight container, for up to 5 days.