In a 10- or 12-inch saute pan, heat the oil to 350 degrees F, verifying the temperature with a candy thermometer.
While the oil heats, spread the peanut butter on one slice of bread, and top it with the sliced banana.
Spread the marshmallow creme on the second slice of bread, and put your sandwich together; the marshmallow creme should be very cold so it doesn't get too runny when fried.
Put the flour, eggs and panko in three separate bowls large enough to fit the sandwich.
Dust the entire sandwich in flour, then dredge it in the eggs, and, with a dry hand, coat it in panko.
Carefully lower the sandwich into the hot oil, and fry until the bottom side is golden brown, about 2 minutes. With tongs or two forks, flip the sandwich and fry on the reverse side until golden brown.
With a spatula, move the sandwich from the pan to a plate. Sprinkle it with salt, and serve.