Preheat grill if using. Place a large cast iron skillet on grill with vegetable oil in it. Heat until almost smoking. Pour in remaining ingredients and cover skillet. Shake skillet gently while wearing an oven mitt, constantly while corn pops. Remove from heat and pour into a large bowl. Don't let it go too long in between pops or the sugar and butter will burn. It's better to have some "old maids" than burnt popcorn.