Put the popcorn in a large bowl. Stir together the melted butter, brown sugar, pumpkin pie spice, salt, and espresso powder in a small bowl. (Use espresso powder to taste. If you can’t find espresso powder, substitute instant coffee granules.)
Drizzle the butter mixture over the popcorn and toss gently until combined. Serve immediately or store in an airtight container for up to 1 day.