Preheat the oven to 250 degrees F. Coat two sheet pans with cooking spray. Place the popcorn in a large bowl.
Cut the butter into 8 slices and place in a saucepan. Place the pan over medium heat.
Melt the butter, then add the brown sugar, corn syrup and salt to the pan.
Increase the heat to medium high, and bring the mixture to a boil, stirring continuously.
Once the mixture comes to a boil, stop stirring and let it cook undisturbed for exactly 4 minutes.
Remove the caramel from the heat. Add the baking soda and vanilla extract, then stir.
Pour the caramel over the popcorn then quickly toss the popcorn in the caramel mixture to coat.
Divide the caramel corn between the two sheet pans. Place the pans in the oven.
Cook for 1 hour. Every 15 minutes, stir the popcorn and rotate the pans from the top to bottom oven racks.
Remove the popcorn from the oven. Cool, then break into pieces and serve.