Preheat the oven to 350ºF and line a baking sheet with parchment paper.
In a large heavy-based pot, melt 2 tablespoons of butter over MEDIUM heat. Place 2 popcorn kernels in the melted butter. Cover with the lid and let the two kernels pop.
Pour in the rest of the kernels and shake the pot to form a single layer. Replace the lid and keep on the heat. Once the popping starts, crack the lid a little to let the steam out.
When there is frequent popping, turn down the heat for a few minutes. Once the popping has subsided, remove the pot from the heat.
Let the popping stop completely before opening the lid.
The Caramel Topping:
In a small pot bring the rest of the butter, brown sugar, syrup, and a pinch of salt to a boil. Be careful the mix doesn't boil over. Boil for 5 minutes or until the sugar has dissolved.
Pour the caramel over the popped popcorn. Toss it all together with a rubber spatula to coat it.
Spread the popcorn out onto the lined baking sheet and sprinkle with the rest of the sea salt. Bake in the oven for 5 minutes and then let it cool before breaking it up and placing it into sealed jars.