Add all of the ingredients to a blender . It blends up much easier if you add the liquid ingredients first, followed by the dry ingredients.
If you don't have a blender you can make them by hand. I will include instructions at the end.
Blend the ingredients until smooth. Don't over blend. turn it off as soon as everything is smooth and combined. It should take less than a minute.
Leave the batter to rest for at least 10 minutes. It will be thick. Don't be tempted to thin it. Grease and preheat your waffle iron while it rests.
Pour/spoon the batter onto the waffle maker and cook on a medium temperature until steam stops coming out of the side. As soon as it does remove them. Ignore any signals from your machine if it tells you they are done earlier.
To make by hand:
Mash the banana really, really well until it is a puree consistency. Beat it together with the milk and oil until combined well.
Add all of the dry ingredients to a bowl, whisk them together, then gradually pour in the milk, stirring as you go to work out the lumps. Stir until all of the flour is absorbed and the batter is smooth, then leave to rest for 10 minutes before carrying on as above.
As always with any recipes involving flour, I highly recommend that you weigh it. Cup measurements are not accurate enough to get the best results. If you do need to use cups, spoon the flour into the cup then level off the top with a knife without compacting it or shaking it down. By doing it like this you will get roughly the correct amount.If you scoop the flour up into the cup, you will end up with much more than is needed and it will affect the outcome of the recipe. Digital scales are available at most big superstores now and you can pick one up for around $10. It's so worth it!These waffles need the little bit of oil in them to have the perfect texture. If you omit it they won't be quite as good. Adding the nut butter as per my substitution suggestion, provides a little bit of fat and helps somewhat for an oil free version, but for the very best results the oil is necessary. Oil is imperative for greasing the waffle iron. Throw leftovers in a freezer bag and freeze for up to 3 months. When you want one remove from the freezer and pop straight into your toaster. If there's a "frozen" button on your toaster push it, if not, they might take 2 cycles to warm right through.