Prepare a work surface by laying out two 3-feet long pieces of parchment paper, overlapping slightly.

After popping the popcorn, transfer it to a bowl. Lift the popcorn with your hands and shake the bowl lightly so any unpopped kernels fall to the bottom.

Lift 4 quarts of popcorn out of the bowl and transfer to a large brown paper grocery bag.

Combine the butter, brown sugars, corn syrup, pumpkin pie spice and salt in a large microwave-safe bowl. Place in the microwave and heat on high power for 2 minutes. Stir to combine. If the butter is not completely melted, return to the microwave for another 30-60 seconds.

After stirring, return to the microwave for 1 minute. The mixture should be bubbling up furiously by this point. Remove bowl and stir well then return to the microwave for another minute. Repeat this cooking and stirring for a total of 3 more minutes.

Remove the bowl from the microwave and stir in the vanilla and baking soda. Stir well until all of the baking soda is well incorporated.

Pour the hot mixture over the popcorn in the bag. Fold the top of the bag down once microwave on high power for 1 minute. Remove the bag and hold it at the top and one of the bottom corners (if needed use a pot holder to hold the bottom corner. Shake well for about 30 seconds then return to the microwave for another minute.

Repeat cooking and shaking two more times for a total of 4 minutes. Turn the popcorn out onto the prepared parchment paper. Allow to cool completely.

Store in an airtight container. The caramel corn will keep well for 1-2 weeks.