Preheat the oven to 250° F. Line 2 baking sheets with parchment paper; set aside.
Pop popcorn using your preferred method. Place the popped popcorn in 2-3 very large bowls. (You will need enough room to toss the popped popcorn with the caramel sauce.)
To make the caramel, melt the butter over medium heat in a 2-quart saucepan. Once melted, stir in the corn syrup, brown sugar and salt. Increase the heat to medium-high and bring the mixture to a boil, stirring frequently.
Once the mixture reaches a boil, let it continue to boil for an additional 5 minutes, without stirring. Remove from the heat and add in the baking soda and vanilla. (The mixture will bubble quite a bit.)
Divide the caramel sauce between the popped popcorn evenly and toss to coat thoroughly using a rubber spatula, salad tongs, or large spoons.
Spread the popcorn evenly onto the prepared baking sheets. Clumps are fine and will spread out during baking.
Bake for 40-50 minutes, tossing and redistributing the caramel sauce every 10 minutes. Test a cooled piece of popcorn after 40 minutes. It should be crispy and if it’s not, return pans to the oven for an additional ten minutes of baking. 8. Remove the baking sheets to a wire rack and allow popcorn to cool completely before packaging. Store in airtight containers.