Preheat your waffle iron and lightly grease it with non-stick spray or a small amount of melted butter.
In a medium bowl, beat the egg until fluffy.
Add the milk, vanilla, melted butter, and sugar to the egg. Whisk until combined.
Add the flour, baking powder, and salt. Stir just until you don't see dry flour. A few small lumps are fine.
Pour enough batter into the waffle iron to cover the surface without overflowing. If your iron is smaller than a Belgian waffle maker, you may have batter left for a second waffle.
Close the iron and cook until the steam stops rising, about 3 to 4 minutes. The waffle should be golden and crisp.
Use tongs to carefully lift the waffle out. Serve immediately.
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Notes
Preheat fully. A properly heated iron is what gives you crispy edges. Most waffle irons take 3 to 5 minutes to fully heat up.Don't overmix. Stir just until the flour disappears. Overmixing develops gluten and makes the waffle tough.Watch the steam. The waffle is done when the steam stops rising from the sides of the iron. Opening it too early tears the waffle.Let the milk warm up. Pull the milk out of the refrigerator 15 minutes before you start. Cold milk seizes the melted butter into small chunks that won't mix evenly into the batter.Waffle maker size matters. This recipe is sized for a Belgian waffle maker. If yours is smaller, you may get 2 waffles from the batter.Grease between waffles if you're making two. Even on a non-stick iron, a quick coating of cooking spray or melted butter between waffles keeps the second one from sticking.