In a small, shallow bowl, mash banana coarsely with a fork. Stir in orange-juice concentrate. Spread the banana mixture over 2 slices of bread and top with remaining 2 slices of bread, forming 2 sandwiches. In a pie plate, whisk together egg whites and milk; add sandwiches and soak for about 20 seconds. Turn sandwiches over and soak for 20 seconds longer. Transfer the sandwiches to a plate. In a small bowl, stir together yogurt and maple syrup or honey. Set aside. In a nonstick skillet, melt half tsp. butter over low heat. Tilt the pan to swirl the butter around the skillet. With a metal spatula, place the sandwiches in the pan, and cook until the underside is browned, 5 to 7 minutes. Lift the sandwiches and add the remaining half tsp. butter. Turn over and cook for 5 to 7 minutes longer, or until browned. Serve with the sweetened yogurt. Serves 2. 302 calories per serving: 4 grams fat, 317 mg sodium and 7 mg cholesterol.