Preheat your oven to 350°F and line your baking sheets with parchment paper. While the oven heats, measure out all your ingredients and set them aside—this ensures smooth mixing without interruptions. Pour the cinnamon sugar mix into a shallow bowl and set it near your workspace for easy access during rolling.

In a large mixing bowl, combine the softened butter, vegetable shortening (or lard), granulated sugar, and packed brown sugar. Beat on medium speed for about 3 minutes until the mixture is light, fluffy, and pale—this aeration is crucial for creating tender cookies with good texture. The extended creaming time helps incorporate air that will make the cookies rise slightly and develop a better crumb.

Add the room-temperature eggs one at a time to the creamed mixture, beating well after each addition to ensure proper emulsion. Pour in the vanilla extract and kosher salt, and mix until everything is fully combined. Room-temperature eggs blend more smoothly and help create a more cohesive dough.

In a separate bowl, whisk together the all-purpose flour, cream of tartar, baking soda, and cornstarch. The cream of tartar reacts with the baking soda to create lift and give snickerdoodles their signature tangy flavor, while the cornstarch adds that characteristic chewy texture. Gently fold the dry ingredient mixture from this step into the wet ingredients from Step 3 until just combined—don't overmix, as this keeps the cookies tender.

Cover the dough and refrigerate for 30 to 60 minutes. Chilling makes the dough easier to handle and prevents the cookies from spreading too much during baking, resulting in thicker, chewier cookies. I find that even 30 minutes makes a noticeable difference in how the dough rolls and holds its shape.

Remove the chilled dough from the refrigerator and roll it into 1-inch balls using your hands or a small cookie scoop. Working quickly so the dough doesn't warm up too much, roll each ball in the cinnamon sugar mix until well coated, then place it on the prepared baking sheets about 2 inches apart. The balls will spread slightly during baking, so spacing is important for even cooking.

Bake at 350°F for 11 to 14 minutes, until the edges are set but the centers still look slightly underdone—they'll continue to cook on the baking sheet after removal. Let the cookies cool on the baking sheet for 2 minutes, then transfer them to a wire rack to cool completely. The cookies will firm up as they cool and will have that perfect combination of crispy edges and chewy centers that defines an excellent snickerdoodle.