Directions:

• Heat the oven to 375°F. Line two baking sheets with parchment paper.
• To make the cookies, whisk the shortening into the warm butter in a large bowl until the shortening melts (if there are still a few little solid bits of shortening, don’t worry). Whisk in the brown sugar and vanilla for 30 seconds, and then the egg. Sprinkle the baking soda, salt, cinnamon, and cream of tartar into the bowl, one at a time, vigorously whisking after each. Gently fold in the flour with a flexible spatula just until the last streak disappears. Don’t overmix.
• To make the cinnamon sugar, whisk together the granulated sugar and cinnamon in a small bowl. Scoop 1  1/2-tablespoon balls of dough with a portion scoop or measuring spoons and drop them in the cinnamon ​sugar, nudging them with your fingers to coat. (No need to roll them into balls first.) Place 9 or 10 on each prepared baking sheet. The dough is super soft. Don’t worry. Bake for 6 to 8 minutes, until the cookies are cracked and slightly domed (they may or may not collapse as they cool—​depends on the temp in your kitchen). Remove from the oven and if too domed/puffy for your liking, immediately press with a spatula to flatten. Enjoy warm or at room temperature. Keep the snickerdoodles in an airtight container on the counter for up to 3 days.