COOKING TIME: 12 MINS
Spread peanut butter evenly on 1 slice of bread, then spread mashed banana on other slice, leaving a 1/4-inch border around edge. Close sandwich, gently pressing bread slices together.
Heat butter in an 8- to 10-inch heavy skillet over moderate heat until foam subsides, then fry sandwich, turning over once, until golden brown, about 2 minutes total.
Eat immediately with a knife and fork.