Trim excess fat from steak and reserve. Sprinkle each side of the steak with salt and pepper and let stand at room temperature for 1 hour. Heat heavy cast iron skillet over high heat. Grease the skillet with the reserved beef fat. Sear the steak quickly on both sides, only 2 minutes per side Reduce heat to med, continue cooking for additional 2 to 3 minutes each side to the desired degree of doneness. You can use an instant-read meat thermometer (125 to 50ºC for medium-rare). Remove the steak to a warm platter and keep warm in a oven while you make a sauce by deglazing the pan. Discard all but 1 tablespoon of fat from the skillet. Put pan back over moderate heat and add the liquid. When liquid boils, scrape pan with a wooden spoon to loosen the good stuff sticking to pan bottom. Continue cooking until liquid is reduced in volume and slightly thickened. Stir in butter. Pour sauce over steak and serve. Makes 1 to 3 servings. Per 4-ounce serving: Calories 240ºC at 36g Cholesterol 126 mg Sodium 325 mg Percent calories from fat 71