Mash banana coarsely with a fork. Stir in orange juice concentrate; then spread over 2 slices of bread and top with the 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 sandwiches for 20 seconds longer. Transfer to a plate. Stir together yogurt and maple syrup or honey. Set aside.
In a nonstick skillet, melt 1/2 teaspoon butter over low heat. With a metal spatula, place the sandwiches in the pan, and cook until the underside is browned. Lift the sandwiches and add the remaining 1/2 teaspoon butter. Turn over and cook until browned.
Serve with sweetened yogurt. Double or triple this recipe depending on the size of your gathering.