Preheat your oven to 400°F and line two baking sheets with parchment paper or silicone baking mat.
In a small bowl, stir together sugar and cinnamon.
In a large bowl, cream together butter, shortening, and sugar.
Add egg and vanilla and beat until well-combined.
Add flour, cream of tartar, baking soda, and salt. Mix until just combined.
Scoop and roll dough into 24 1 1/4-inch balls. Roll in cinnamon sugar mixture and place on baking sheets. Bake sheets one at a time for 8 to 10 minutes, until cookie tops are cracked and just set.
Cool for 5 minutes on the baking sheet before transferring to a cooling rack. Serve and enjoy!
Cookies will stay fresh in an airtight container at room temperature for 4 to 5 days or in the freezer for up to 2 months.*If using unsalted butter, double the salt to 1/4 teaspoon.*The shortening in this recipe can be replaced with the same amount of butter (for 1/2 cup of butter total). *Correct flour measurement is very important in this recipe. For best results, measure by weight or the Spoon and Sweep Method (stir the flour in your container to fluff it up and then spoon it into your measuring cup, sweep excess off with the flat of a knife).To Freeze Dough: After rolling cookie dough balls in cinnamon sugar, place them on a plate or small baking sheet that will fit in your freezer and freeze until solid. After dough is frozen, transfer to an airtight container or freezer bag and store in the freezer for up to 2 months.To Reheat Frozen Cookie Dough: Preheat oven to 400°F and place frozen cookie dough on a cookie sheet lined with parchment paper or silicone baking mat. Bake for 8 to 11 minutes.