Toss the popcorn and cashews together in a very large, heat-proof mixing bowl. Spray two half sheet pans with non-stick cooking spray or line with silpats.

Preheat the oven to 250°F.

In a large saucepan combine the brown sugar, butter, dark maple syrup, vanilla, and kosher salt over medium heat. Stir until the butter is melted. Bring to a boil. If using the bourbon, add it when the mixture reaches a boil. Boil for 5 minutes, remove from the heat, and stir in the baking soda. It will foam up and expand. When the baking soda is fully dissolved and the caramel is foamed up, pour it over the popcorn and cashews in the mixing bowl and gently toss until everything is evenly coated. Divide between the two half sheet pans and bake for 1 hour, stirring and rotating pans every 15 minutes. Let the caramel corn cool on the silpats (or gently transfer into a heat-proof mixing bowl to cool if using sprayed pans.) This caramel corn is good for a week or so in a tightly lidded container at room temperature.