In a blender, combine the bananas, milk, and melted butter. Blend until smooth. Add the flour and blend again until you have a smooth batter.
Grease a non-stick pan with a little butter or oil and place it over medium heat.
Pour some batter into the pan and tilt the pan to spread the batter evenly, covering the bottom. Cook for about 2 minutes, or until the edges start browning and bubbles form on the surface. Using a spatula, carefully flip the crepe and cook for another minute, or until it is cooked through.
Remove the crepe from the pan and repeat the process with the remaining batter. Spread peanut butter and banana slices, or any toppings of your choice, onto the crepes before serving.