Preheat your oven to 400°F and line two baking sheets with parchment paper or silicone baking mat.
In a large bowl, cream together butter, shortening, and sugar.
Add egg and vanilla and beat until well-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.
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.
*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).
*If using unsalted butter, double the salt to 1/4 teaspoon.