Preheat oven to 200℉
Over medium heat, combine the butter, corn syrup, and brown sugar for 10 minutes. Bring the mixture to a boil while stirring regularly.
Remove the pan from the heat
Add in the vanilla extract and baking soda (and watch the caramel become puffy!)
Pour puff corn into a large baking dish
Pour the caramel mixture over the puff corn kernels
Gently stir and coat the popcorn kernels with caramel (be careful, the caramel is hot!)
Bake for 1 hour, stirring every 15 minutes
Prepare a flat surface with wax paper or parchment paper
Pour the caramel puff corn onto the wax paper and separate the caramel corn pieces. Because the popcorn is cooking at such a low heat, you should be able to do this by hand, but be aware of the temperature and use utensils if necessary
Allow the separated caramel popcorn to cool for 30+ minutes, then enjoy!
Store the caramel puff corn in an airtight container