Preheat waffle iron. Mine just turns on, but if there are temperature settings choose HIGH.
In a medium mixing bowl, beat the egg white until soft peaks form. (I start my egg whites in the stand mixer and let them go while I mix everything else together.)
In another medium bowl, stir together flour, baking powder, and salt. Set aside.
In a large bowl, beat together the egg yolks and sugar until the sugar is completely dissolved and eggs have turned a pale yellow. Add vanilla, melted butter, and milk to the eggs and whisk to combine. The mixture will look broken, that's okay.
Pour the flour mixture into the egg yolk mixture and whisk until just blended. Do not over mix. Using a rubber spatula, fold the egg whites into the waffle batter until they're just almost incorporated. Do over mix.
Coat the inside of your waffle iron with non-stick cooking spray. Pour enough batter in the iron to just cover the waffle grid. I use about 3/4 cup in my waffle maker.
Close and cook according to the waffle makers instructions until browned, about 5 to 7 minutes. If the indicator comes on and your waffle isn't a nice crispy golden brown, close the iron and let to cook for another minute or two.
Remove waffle from the iron. Repeat with the remaining batter. You can keep the cooked waffles warm in your oven at 200 degree F. When ready to serve, spread with butter and top with syrup. Serve immediately.
If you don't have cake flour on hand, you can substitute 1 3/4 cups all-purpose flour plus 1/4 cup cornstarch.