Place a large, heavy pot or Dutch oven on medium heat to warm up. Pour in 3 tablespoons of the butter. Add 2 kernels, place the lid on, and listen for them to pop.
Once the two kernels have popped, add the rest of the popcorn. It should cover the bottom of the pot in a single layer. Cover and listen for the popping to get really rapid. This takes 3-4 minutes. Place the lid slightly ajar to let out some of the steam. When the popping starts slowing down, turn off the heat.
While the popcorn is still hot, pour over the rest of the melted butter and sprinkle on the salt. Toss well to coat.