Pop the popcorn and place it in a large mixing bowl. Remove any unpopped kernels.
Melt the butter in a small saucepan over low heat.
Add the brown sugar, pumpkin pie spice, and salt. Stir until the sugar begins to dissolve and the mixture is well combined.
Remove the saucepan from the heat and stir in the vanilla extract.
Slowly drizzle the warm pumpkin spice mixture over the popcorn while gently tossing to coat it evenly.
Spread the coated popcorn onto a parchment-lined baking sheet in an even layer.
Allow the popcorn to cool completely so the coating can set and become crunchy.
Break apart any large clusters and serve immediately or store in an airtight container.