Line two large baking sheets with parchment paper or silicone baking mats. Preheat the oven to 250°F (120°C).

Pop about 1/2 cup popcorn kernels using an air popper, stovetop method, or a plain microwave popcorn bag. Measure 10 cups of popped popcorn into a very large mixing bowl.

Gently scoop the popcorn into the bowl by hand or shake the bowl so unpopped kernels fall to the bottom. Discard any unpopped kernels so they don’t end up in the finished snack.

Sprinkle the popcorn with 1/2 teaspoon fine sea salt and gently toss until lightly and evenly salted.

In a medium saucepan over medium heat, combine the unsalted butter, granulated sugar, light brown sugar, light corn syrup, cream of tartar, and ground nutmeg (if using). Stir until the butter is melted and the mixture is well combined.

Bring the mixture to a steady bubble over medium heat, stirring often. Once it is bubbling all over the surface, cook for 4 minutes, stirring occasionally, just until the sugar is dissolved and the syrup is glossy. Do not overcook to hard caramel.

Remove the saucepan from the heat. Stir in the ground cinnamon and vanilla extract until fully incorporated. The mixture may bubble briefly when the vanilla is added.

Carefully pour the hot cinnamon sugar mixture over the salted popcorn in the large bowl. Using a heat-safe spatula or wooden spoon, gently toss and fold the popcorn from the bottom up until the kernels are as evenly coated as possible. Some clumps are fine at this stage.

Divide the coated popcorn between the two prepared baking sheets. Spread into as even a layer as possible, breaking up any very large piles so it bakes and dries evenly.

Bake the popcorn at 250°F (120°C) for 25–30 minutes, stirring and re-spreading the popcorn every 10 minutes. The popcorn is done when it feels drier to the touch and smells deeply buttery and cinnamon-y.

Remove the baking sheets from the oven and let the popcorn cool completely on the pans. As it cools, it will crisp up more. Once fully cool, break apart any large clusters with your hands.

Taste the cooled popcorn. If you’d like more cinnamon sugar flavor, toss it with 1–2 teaspoons of additional cinnamon sugar (about 1 tablespoon sugar mixed with 1/4 teaspoon cinnamon) while it is still just slightly warm, then let cool completely before storing.