Season steak with salt and pepper and place the steak in the refrigerator uncovered for at least an hour. Preheat grill to medium on one side. Place steak on the side of the grill that is not directly heated, and place a cast iron skillet over the direct flames. Flip every 3 minutes, and remove the steak once it has reached a temperature 20 degrees less than desired (e.g. medium is about 135, so remove the steak once it reaches 115). Increase heat to high on all burners. Place butter in skillet and let is melt for about 5 minutes. Place steak on the skillet, and cook for 1 minute on each side. Let it rest for 5 minutes.