Preheat the oven to 225 degrees F, and line a sheet pan with parchment paper. Set the popcorn in a large bowl and set aside.
In a medium saucepan set over medium-high heat, bring the butter, sugar, molasses, corn syrup, pumpkin pie spice and salt to a boil. Stir briefly with a heatproof spatula, then continue boiling for three minutes without stirring.
Remove from the heat and immediately pour over the popcorn. Stir until evenly combined.
Spread the popcorn evenly onto the sheet pan, and then place in the oven. Cook for one hour, stirring every 15-20 minutes.
Remove from the oven, cool briefly to set, and then break into pieces (or enjoy as clusters). Serve warm or at room temperature.