1 cup packed light or dark brown sugar
1/4 cup balsamic vinegar
2 tablespoons Worcestershire sauce
3 medium cloves garlic, coarsely chopped
2 tablespoons coarsely chopped fresh rosemary
1-1/2 lb. skirt steak, trimmed of excess surface fat, soaked and rinsed in cold water for at least 20 minutes
Ground black pepper
Combine the sugar, vinegar, Worcestershire, garlic, and rosemary in a baking dish or zip-top bag large enough to hold the steak. Add the steak and turn to coat. Let sit at room temperature for at least 5 minutes and up to an hour.
Prepare a high (500°F to 600°F) charcoal or gas grill fire. Take the steak out of the marinade and season both sides with pepper. Cut into smaller pieces to make grilling more manageable, if you like.
Grill, flipping once, until grill marks form on both sides and cooked to your liking, about 3 minutes per side for medium rare (135°F). Transfer to a cutting board and let rest for 5 minutes. Slice against the grain and serve.