Place butter, sugar and brown sugar in a large mixing bowl and beat until creamy. Add egg, egg yolk & vanilla and mix until thoroughly combined.
In a separate bowl, mix together flour, cream of tartar, salt, baking soda, and cinnamon until blended. Add dry mixture into wet ingredients and mix until just combined.
Cover bowl and place into refrigerator for 1 hour.
Preheat oven to 350 degrees.
Make sweet coating by combining sugar and cinnamon into a small bowl.
Remove dough from refrigerator and form into about 20-22 rounds. Roll into sweet coating until completely covered and place on a parchment lined baking sheet.
Bake for about 11-12 minutes, remove and let cool on a baking sheet for 2 minutes, then move to cooling rack.
We like them slightly undercooked, so depending on how you like them and the accuracy of your oven, feel free to bake them a minute more or less. I normally use 2 baking sheets, so that when the first batch comes out I am ready to add the next batch into the oven. Enjoy!