Preheat your oven to 250°F (120°C). Line a large rimmed baking sheet with parchment paper or a silicone baking mat to prevent sticking and make cleanup easy.
Pop the popcorn using an air popper, stovetop method, or plain microwave popcorn. Measure out about 10 cups of popped popcorn and place it in a very large mixing bowl. Pick out and discard any unpopped kernels.
10 cups popped popcorn
In a small bowl, whisk together the granulated sugar, ground cinnamon, cream of tartar, salt, and brown sugar if using. Whisk until the cinnamon is evenly distributed with no clumps.
1 cup granulated sugar, 2 teaspoons ground cinnamon, 1/4 teaspoon cream of tartar, 1/2 teaspoon kosher salt, 1-2 tablespoons brown sugar
In a small saucepan over medium-low heat, melt the butter until completely liquid and just starting to foam. Do not brown it. Remove from the heat and stir in the vanilla extract.
1/2 cup unsalted butter, 1 teaspoon pure vanilla extract
Pour the cinnamon sugar mixture into the warm melted butter and stir until it forms a thick, sandy-looking paste. It may look grainy; just make sure everything is evenly moistened.
1/2 cup unsalted butter, 1 cup granulated sugar, 2 teaspoons ground cinnamon, 1 teaspoon pure vanilla extract, 1/4 teaspoon cream of tartar, 1/2 teaspoon kosher salt, 1-2 tablespoons brown sugar
Pour about half of the cinnamon butter mixture over the popcorn. Gently toss with a spatula or clean hands, reaching down to the bottom of the bowl to coat evenly. Add the remaining mixture and toss again until most of the popcorn is coated, leaving some lighter pieces for variety.
10 cups popped popcorn, 1/2 cup unsalted butter, 1 cup granulated sugar, 2 teaspoons ground cinnamon, 1 teaspoon pure vanilla extract, 1/4 teaspoon cream of tartar, 1/2 teaspoon kosher salt, 1-2 tablespoons brown sugar
Transfer the coated popcorn to the prepared baking sheet. Spread it into a fairly even layer, breaking up any large clumps so it can crisp evenly.
10 cups popped popcorn
Bake for 35–45 minutes, stirring every 10–15 minutes. Each time you stir, spread the popcorn back into an even layer. The goal is to dry and crisp the coating, not to brown it. Test a few pieces: when they cool and crunch cleanly (not chewy), it’s done.
Remove the baking sheet from the oven and let the popcorn cool completely on the pan. It will continue to crisp as it cools. Break up any clusters with your hands.
Taste a handful once fully cool. If you’d like more cinnamon flavor, sprinkle a small amount of additional cinnamon sugar (equal parts sugar and cinnamon) over the top and toss again.
Serve the snickerdoodle popcorn in a big bowl, or portion it into treat bags, jars, or tins for gifting. Store any leftovers in an airtight container at room temperature.