Make Popcorn: In a large saucepan, heat 2tbsp oil and couple of corn kernels. Cover and once the corn in the pan starts to pop, add the remaining corn, cover and let all of the corn pop. Turn off the heat once the popping subsides, remove the popcorn into a large mixing bowl.
Add nuts (peanuts or almonds) to the popcorn and mix well.
Preheat the oven to 250°F.Grease a large baking sheet and set aside.
Measure out baking soda and vanilla extract and keep ready. Once the caramel gets going, it will be a very quick process and you don't want to leave it unattended for too long -- so better to keep all the ingredients ready and handy.
Make the caramel: In a large saucepan, combine butter, brown sugar and corn syrup. Let the sugar melt on low flame, then increase the flame to medium and let the caramel boil while stirring continuously to prevent sticking to the bottom. Boil for 4~5 minutes.
Take it off the heat and stir in baking soda and vanilla. Stir vigorously. The mixture will become bubbly and frothy -- make sure to use a large enough pan to avoid spill over.
Pour the caramel over the popcorn and sprinkle the sea salt on top. Mix everything to combine. It is not possible to coat popcorn evenly because caramel thickens as it cools. But once baked, it will be easy to mix.
Transfer the popcorn mixture into the prepared baking pan and bake for 1 hour. Every 15 minutes, remove the baking sheet from the oven and give the popcorn a good mix.
Remove the popcorn from the oven and let cool. Store in an airtight container and enjoy!!