Preheat the waffle iron. Mash the ripe banana with a fork on a plate or in a zip bag until it has a pureé-like texture.
Whisk together the flour, baking powder, salt, and sugar in a large bowl.
Add the milk, egg, butter, mashed banana, and vanilla into the bowl. Mix until mostly smooth, with few remaining lumps.
If needed, this varies by waffle iron, grease it right before pouring the batter in. Cook the waffles until steam stops coming out of the sides; this is when you can open and check. Once done, remove the waffle from the waffle iron and repeat the process with the remaining batter.
Serve the waffles topped with additional sliced bananas, chopped walnuts, and syrup.
Leftover waffle batter can be kept in the fridge for up to 3 days. Bring it to room temperature before cooking it. Cooked waffles can be refrigerated for up to 4 days or frozen for up to 3 months.
