Pour the oil into a large pot with a lid. Add a few popcorn kernels, cover the pot, and place over medium-high heat. As soon as you hear them pop add the remaining kernels and cover the pot again. Cook, shaking the pot occasionally, until the popping slows almost to a stop, 3 to 5 minutes.

Remove the pot from the heat and pour the butter over the popcorn. Sprinkle with about 2/3 of the buttermilk mixture. Cover the pot and give it a good shake to coat the popcorn evenly. Taste and add a little more of the buttermilk mixture if you want more salt and flavor.