- Preheat your oven to 400°F. Cover two cookie sheets with parchment paper or a silicone baking mat. 
- In a stand mixer, using the paddle attachment, cream butter, shortening, and sugar. Beat in egg and vanilla. 
- Scrape down the sides and bottom of the bowl. On low, mix in flour, cream of tartar, baking soda, and salt until just combined. 
- In a small bowl, mix 2 tablespoons of sugar with the cinnamon. Scoop 36 1/2-ounce mounds of dough into balls (just over 1-inch wide) and roll in sugar mixture. Place on a baking sheet 1 1/2-inches apart. These are small cookies, so you should be able to fit 18-20 on a standard-size cookie tray. 
- Bake for 6 to 8 minutes. They are done when they are just golden and look set. You don't want them to crisp up too much since we are turning them into sandwiches. 
- Allow cookies to cool completely.