Brush the tenderloin filets on all sides with 1 tablespoon olive oil, then sprinkle with sea salt and cracked pepper. Heat another tablespoon of olive oil in a skillet over medium-high heat until it begins to smoke. Add the steaks, and cook to your desired degree of doneness, 4 minutes per side for medium-rare. Once done, remove the steaks from the skillet and keep warm.
Stir in the minced shallot, and cook for 1 minute. Pour in the port and balsamic vinegar. Simmer until the port reduces to the consistency of chocolate syrup, 2 to 3 minutes. Add the beef stock and rosemary, and return to a simmer. Once the sauce has begun to simmer, whisk in the chocolate until it melts and slightly thickens the sauce. Pour sauce over steaks to serve.