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.
Make the batter:
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.
Cook:
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:
Serve the waffles topped with additional sliced bananas, chopped walnuts, and syrup.
Storage:
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.