Lay out the bread slices and spread them both with peanut butter. Lay the banana slices on one piece of bread. Press the 2 slices together.
Butter one side of the sandwich and put it on a hot panini maker butter-side down. Butter the other side, close the lid and cook until it's toasted and warmed through. (If you do not have a panini maker, you can grill the sandwich in a skillet, laying another heavy skillet on top of the sandwich to press it together. Turn to grill the other side in the same way.)
Sprinkle with powdered sugar, cut in half and serve!