Preheat the oven to 350ºF and line a baking sheet with parchment paper.
In a large heavy-bottomed pot, melt 2 tablespoons of butter over medium heat. Place 2 popcorn kernels in the melted butter. Cover with the lid and let the 2 kernels pop.
After the 2 kernels have popped, pour the rest of the kernels into the pot and shake the pot to form a single layer. Replace the lid. 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, with a few seconds between pops, remove the pot from the heat.
Let the popping stop completely before opening the lid.
The Caramel Topping:
In a small saucepan, bring the rest of the butter, brown sugar, syrup, and a pinch of salt to a boil over medium heat. Be careful the mixture doesn't boil over. Boil for 5 minutes or until the sugar has dissolved.
Pour the mixture over the popped popcorn. Toss it all together with a rubber spatula until the popcorn is well-coated.
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, then let it cool before breaking it up. Place the popcorn into jars or containers with airtight lids.