Pop the popcorn. Brown the butter: Melt the butter in a skillet or small saucepan over medium heat. Swirl or stir the butter with a wooden spoon as it starts to foam and sputter. Remove the butter from the heat as soon as it begins to turn golden brown and smells nutty, about 1 minute. Drizzle over the popcorn. Sprinkle with the Parmesan and salt and toss well.