In a large mixing bowl, whisk together the flour, baking powder, sugar, and salt. Set aside.

In a high speed blender add milk, egg, vanilla extract, banana, and spinach. Blend on high until completely smooth and there are no more chunks of spinach or banana.

Whisk the wet ingredients into the dry ingredients and mix until just combined and there are no more white streaks. Be careful not to overmix, as this can result in tough waffles.

Preheat your waffle maker according to the manufacturer's instructions. Once heated, lightly grease the waffle maker with cooking spray or oil.

Use a ladle or ½ cup measuring cup to pour the batter into the waffle maker. Cook according to the manufacturer's instructions and desired done-ness.

Use a spatula to carefully remove the waffles from the waffle maker and transfer them to a plate or cooling rack. Repeat with the remaining batter.

Serve the spinach waffles warm with your favorite toppings, such as fresh fruit, whipped cream, or maple syrup.