Whisk to combine flour, baking powder, and salt in a large bowl.
Combine eggs, milk, melted butter, mashed bananas, maple syrup, and vanilla extract in a medium bowl. If milk is not at room temperature, heat it in the microwave for about 30 seconds before adding.
Stir wet ingredients into dry ingredients until just combined. There will be a few lumps remaining, but do not over-stir.
Spray the waffle iron with non-stick cooking spray or brush it with vegetable oil to prevent sticking according to your waffle iron instructions. Pour batter into the center of the waffle iron and spread to the edges with the back of a spoon or spatula. Use approximately 1 cup of batter for a large Belgian waffle iron. Cook until golden brown.
Serve immediately or place waffles on top of a wire rack in a single layer in a 200-degree oven with the door slightly open to keep them warm and crisp until ready to serve.
Notes
All-purpose flour will not give the best results. Cake flour can be made from scratch from all-purpose flour by removing 2 tablespoons of flour from each cup, replacing it with 2 tablespoons of cornstarch, then whisking it together.