Preheat oven to 250 degrees F. Place the popcorn in a large roasting pan (I like using a disposable aluminum pan). If using a metal pan, lightly grease the pan.
Melt the butter, brown sugar, corn syrup and salt together in a medium saucepan over medium heat. Bring the mixture to a boil and boil for 5 minutes, stirring constantly to prevent scorching. Remove from the heat and whisk in the vanilla extract, baking soda, and pumpkin pie spice.
Pour the hot caramel mixture evenly over the popcorn in the pan, using a spatula to toss and coat the popcorn evenly. Note that the caramel mixture may not cover all of the popcorn off the bat, but as you bake it, the caramel will thin out to coat nearly completely.
Bake for 1 hour, stirring every 15 minutes to coat the popcorn evenly. Cool the popcorn mixture in the pans completely before breaking into pieces to serve.