For each sandwich, place one-half of a good-quality 3-1/2-ounce chocolate bar on one slice of white or wheat bread. Top with 1/2 of a banana, sliced, and second slice of bread. Heat 1 tablespoon butter over medium-low heat just until it begins to bubble. Add sandwich to skillet, chocolate side down, and cover skillet with lid. Cook until bread is golden brown and toasted, about 1-1/2 minutes. Carefully flip sandwich over, and repeat. Remove from skillet, slice in half and dust with sifted powdered sugar. Serve warm. Makes 1 serving.