Preheat the waffle iron according to the manufacturer's instructions.
In a mixing bowl, combine the mashed bananas, egg, and milk. Whisk until smooth.
In another bowl, mix the flour, baking powder, salt, and sugar.
Gradually add the dry ingredients to the banana mixture, stirring gently.
Fold in the melted butter and vanilla extract until just combined.
Lightly grease the waffle iron with cooking spray or butter.
Pour the batter onto the preheated waffle iron, using about 1/2 cup per waffle.
Close the lid and cook until golden brown, about 3-5 minutes.
Carefully remove the waffles and serve warm.