Evenly spread peanut butter onto one slice of bread, and top with banana slices. Top with the remaining bread slice. Set aside.
Bring a skillet sprayed with nonstick spray to medium-high heat on the stove.
Evenly spread 1/2 tablespoon of butter on top of the sandwich.
Add sandwich to the skillet with the buttered side down. Carefully spread remaining 1/2 tablespoon of butter onto the top slice of bread. Cook until lightly browned on both sides, about 2 minutes per side.