Preheat oven to 250F (120C). Line a cookie sheet with aluminum foil and lightly grease with cooking spray.

Place popcorn, on prepared baking sheet and place in the oven on the center rack to keep warm while you prepare the caramel coating.

Place butter in a medium-sized saucepan over medium heat. Cook until butter is melted.

Add brown sugars, corn syrup, and salt and cook until the sugar is dissolved.

Bring mixture to a boil (still over medium heat), Boil for 3 minutes, stirring frequently.

Remove caramel from heat and stir in vanilla extract and baking soda until completely combined.

Remove popcorn from oven and drizzle caramel over popcorn. Sprinkle nuts over top (if using) and then use a spatula to stir until popcorn and nuts are mostly coated with caramel (it’ll be a little tough to get popcorn completely coated, aim for coating about 75% of the popcorn, you’ll be able to coat more later on).

Return cookie sheet to center rack in oven and bake on 250F (120C) for 40-45 minutes, removing the popcorn every 10-15 minutes to stir.

Once finished cooking, stir again and allow popcorn to cool completely on the baking sheet before enjoying.