Steak au Poivre with Chocolate Balsamic Reduction

Pat steaks dry and coat both sides with peppercorns, pressing to adhere. Season with sea salt. Heat oil with 1 tablespoon butter in a 12-inch heavy skillet over moderately high heat until hot but not smoking. Reduce heat to moderate and cook steaks, about 4 minutes on each side for medium-rare. Transfer steaks to a platter. Add vinegar to skillet and deglaze by boiling over high heat, scraping up brown bits. Simmer vinegar until reduced to about 1/4 cup. Remove from heat and whisk in remaining tablespoon butter until melted. Season sauce with salt and drizzle over steaks.