Place the popped popcorn in a large bowl and set it aside.
In a medium saucepan over medium heat, melt the butter. Stir in the brown sugar, pumpkin puree, and salt until the mixture is smooth and bubbling.
Let the caramel cook for 3 minutes without stirring, then remove the pan from the heat.
Quickly stir in the vanilla extract, pumpkin pie spice, and baking soda. The mixture will foam slightly.
Pour the hot caramel over the popcorn and stir gently until all the kernels are coated.
Spread the coated popcorn onto a parchment-lined baking sheet and let it cool for 20 minutes so the clusters can set.