The deep and rich flavors of the unique steak marinade in this Coffee Balsamic Flank Steak seep in over many hours, making it a truly special meat dish, with rave reviews. note that almost all of the prep time is inactive, marinating time.
Cut any excess fat off of the steak and place it in a large zip-lock bag -- the bag should be large enough so the steak is flat, or almost flat. Pour the marinade in the bag, making sure it coats both sides of the meat. Gently let all the air out of the bag, seal it, and place it in the refrigerator overnight and ideally for about 24 hours. (It's a good idea to put the bag in a bowl in case it leaks.)
Season both sides generously with salt and pepper.
While the steak is grilling, add the coffee syrup or maple syrup, balsamic vinegar and espresso powder to a small saucepan, and place it over medium heat. Once it's bubbling, turn the heat to low and let it simmer until it thickens slightly and reduces a bit, about 3 minutes. Then add the butter, and swirl the pan around until the butter has completely melted and blended in. Season to taste with salt and pepper.
*If you don't have a grill, you can also sauté it. A cast iron skillet would be best for this, and you can follow the same cooking directions for the grill.