Prepare an outdoor grill for medium-high heat.
Combine the brown sugar, espresso powder, ancho powder, chili powder, paprika, cayenne, 2 teaspoons salt and ¼ teaspoon black pepper in a small bowl.
Finely grate the garlic or put it through a garlic press to make a paste. Rub the steaks all over with the garlic. Pat the spice rub on both sides of each steak (but not on the outside fat cap). Let the steaks sit at room temperature for 20 minutes.
Grill until grill marks appear and the steaks are nicely browned (for crisscross grill marks, turn the steaks 90 degrees after about 2 minutes), 5 to 6 minutes per side for medium rare. Transfer the steaks to a serving platter and top each with ½ tablespoon of butter. Let the butter melt and the steaks rest for 5 minutes before slicing. Serve with the collected juices.