Heat a 10-inch cast-iron skillet over high heat until very hot. Season steaks on both sides with salt and pepper. Add 1/2 teaspoon butter to one side of pan, and immediately place steak on top; repeat with remaining butter and steak. Do not move meat. Turn after 2 minutes, and cook until an instant-read thermometer reaches 125 degrees for medium-rare or 130 degrees for medium.
Transfer steaks to warmed plates to rest; reserve skillet for pan sauce, if desired.