Preheat oven to 250°. Pop corn and remove any unpopped kernels. Transfer to buttered roasting pan. Dust each batch of popcorn with salt and pumpkin spice mix.
Once popcorn is finished-begin caramel.
In a medium saucepan over medium heat, melt butter, stir in brown sugar, corn syrup, 4 tablespoons pumpkin spice, and salt. Bring to a boil, stirring constantly.
Once it boils, stop stirring and set timer for 4 minutes.
Remove from heat and stir in soda and vanilla.
Pour half over popcorn in one pan and finish pouring the rest over second pan. Stir gently to coat.
Bake pans of coated popcorn in preheated oven, stirring every 15 minutes, for 1 hour. Sprinkle with sea salt lightly, each time you stir. Remove from oven and gently stir to separate kernels as it cools.
Keep in a sealed bag or container for up to 2 weeks... but it won't last that long!