1. Preheat oven to 325 degrees. Line two baking sheets with parchment paper.
2. In the bowl of a stand mixer, or a medium bowl using a hand mixer, cream the butter and sugars until very light and fluffy (about 3 minutes). 
3. Beat in eggs and egg yolk, one at a time. Stir in vanilla. 
4. In a separate bowl, whisk together the flour, baking soda, cream of tartar, salt, and 1 tsp cinnamon (if using). 
5. With the mixer on low speed, slowly add the flour mixture to the butter mixture until thoroughly combined. (If your dough seems warm and slightly loose – or if it works better for your schedule – you can chill the dough at this point, otherwise it’s not necessary). 
6. Using a small cookie scoop or a spoon, shape the dough into small balls (I use a one-TB scoop for this). It’s important to keep the balls of dough relatively small in order to avoid spreading. Larger dough balls may result in wider, slightly flatter cookies (still good, but not as puffy). 
7. Roll each ball in the cinnamon sugar and place on the parchment-lined baking sheet, keeping about 2 inches in between cookies. 
8. Bake at 325 degrees for 10-12 minutes, until the edges of the cookies are set and the middles look slightly underdone. 
9. Cool for a few minutes on the sheet, then transfer to a baking rack to cool completely. 

Cookies will keep for several days in an airtight container.