Preheat your oven to 300 degrees F. Line a large rimmed baking sheet with parchment paper or a silicone baking mat and set it aside.

Pop your popcorn and place it in a very large mixing bowl. Pick out any unpopped kernels. You need the bowl to be large enough to toss the popcorn without it spilling over the sides once you add the caramel.

Melt the butter in a medium saucepan over medium heat. Once melted, add the brown sugar, corn syrup, cinnamon, and salt. Stir to combine, then let the mixture come to a full rolling boil. Once it boils, stop stirring and let it cook for exactly 4 minutes. The caramel will darken slightly and smell toasty.

Pull the pan off the heat and immediately stir in the baking soda and vanilla extract. The mixture will bubble up and foam, which is normal. Stir quickly until it is uniform. This step lightens the caramel so it coats evenly.

Pour the hot caramel over the popcorn in the bowl right away. Using a wooden spoon or heat-safe spatula, fold and toss the popcorn until every kernel has a coating of caramel. Work quickly before the caramel starts to firm up.

Spread the coated popcorn in a single layer on the prepared baking sheet. Slide it into the oven and bake for 15 minutes, stirring gently once at the 7-minute mark to help even out the coating.

Remove the pan from the oven and let the popcorn sit on the pan for 2 full minutes before you touch it. The caramel is still setting during this time. After 2 minutes, use a spatula to gently break any large clumps and spread the popcorn back into a single layer to cool completely, about 10 more minutes.

Once completely cool and crunchy, transfer to a serving bowl or divide into bags. It is ready to eat right away.