Start by preheating your oven to roughly 250 degrees F (95 degrees C). While the oven is heating up, you can work on prepping the caramel popcorn.
Now, place all the popcorn in a bowl – make sure it is a large one, so you have plenty of space to mix the whole thing up as you add in the ingredients.
Place a medium-sized saucepan on the stove over medium heat. Put in the butter and allow it to melt completely. Once it has melted, add in the brown sugar, the corn syrup, and the salt. Keep stirring continuously, until you bring the butter up to a boil.
Now, allow the butter to boil without stirring it for around 4 minutes. Then, remove it from the heat, and stir in the baking soda and the vanilla extract. Your caramel is now ready.
Pour this mixture all over the popcorn in the bowl, and toss it to coat it evenly with the caramel. Once this is done, spread the popcorn out onto two shallow baking dishes covered with parchment paper.
Slide these dishes into the preheated oven, and allow them to bake for around an hour, stirring the popcorn after every 15 minutes. After the hour is up, remove the dishes from the oven, and place them on a counter to cool. Once they have cooled completely and hardened somewhat, you can break them into pieces, and serve.