Prepare a baking sheet with parchment paper and preheat oven to 250 degrees.
In a medium saucepan, melt butter over medium heat.
Mix in boiled apple cider and brown sugar. Stir with a rubber spatula until the sugar is fully combined. Increase heat to medium-high, bringing the mixture to a boil. Let mixture boil for about 4 minutes, while continuously stirring.
Remove caramel from the heat and stir in vanilla, salt, spice, and baking soda. The mixture will start to bubble up, but continue stirring until the sauce is thick and shiny.
In a large bowl, pour your caramel sauce over the popcorn while stirring. Continue stirring in caramel sauce until all of the popcorn is coated.
Evenly spread popcorn onto baking sheet. Bake for an hour, making sure to stir and break up any clumps every 15 minutes during the baking process.
Let popcorn cool. It can be eaten immediately and stored for about a week.
Notes
To make my own boiled apple cider, I brought about 4 cups of apple cider to a boil and reduced it to a simmer until it was 1/8 of its initial volume. The reduced cider should be syrupy and will thicken as it cools. Recipe inspired by The Kitchn.