In a small bowl combine brown sugar, espresso powder, paprika, 2 teaspoons of the salt, the pepper, cumin, and coriander; rub spice mixture all over steaks, patting until evenly coated. Set aside.
In a small saucepan melt butter over medium heat; stir in honey, vinegar, and the remaining 1/4 teaspoon salt.
For a charcoal or gas grill, place plantains, cut sides down, on the rack of a covered grill directly over medium heat. Grill for 10 minutes. Turn cut sides up and brush with honey mixture. Add steaks to grill. Cover and grill for 6 minutes. Brush plantains again with honey mixture and turn steaks. Cover and grill for 6 to 10 minutes more or until plantains are fork-tender and steaks are desired doneness (145 degrees F for medium rare or 160 degrees F for medium).
Cover steaks; let stand for 5 minutes. Serve plantains with steaks.