Spread peanut butter side to side, evenly on 1 side of each piece of bread. Slice banana in 3/4-inch slices and place on 1 piece of bread with peanut butter. Banana slices should be close together, and cover 1 side completely (usually 9 slices). Top with the other slice of bread.
Heat butter in saute pan on medium heat. Place sandwich in pan and brown for approximately 3 to 5 minutes or until golden brown. Add 2 ounces of butter to the pan and flip sandwich to other side and continue cooking for 2 to 5 minutes or until golden brown and peanut butter is melting. Cut into quarters and serve very hot.