Have eggs and milk at room temperature for optimum fluffiness.
Preheat waffle iron. (If yours has multiple doneness settings, try about ¾ of the way from light to dark.)
Sift together flour, sugar, baking powder, and salt into a large mixing bowl.
Separate egg whites from yolks. Place whites into a medium mixing bowl and set aside for the moment. Add yolks to milk. (I like to use a 2-cup glass measuring cup to measure the milk and then mix together the wet ingredients right in the measuring cup.)
Add vanilla to milk mixture and whisk together until well combined.
Pour in slightly cooled melted butter and whisk to thoroughly.
Pour wet ingredients over dry ingredients and stir together until just combined. Some lumps of flour are fine. Don’t over-mix.
Let batter rest while you whip the egg whites.
Whip egg whites with an electric mixer to the stiff peak stage. This means they are white and shiny, and if you turn off the mixer and pull it out of the egg whites, the little point at the top of the whites stands straight up. There is such a thing as overbeating egg whites, so stop once you reach this stage.
Scrape egg whites into the bowl with the batter and use a silicone spatula to gently fold the whites into the batter. (See note 2 below.) Leave some streaks of whipped egg white throughout the batter. Don’t over-mix or the egg whites will deflate too much.
Spray waffle iron with nonstick cooking spray, or brush very lightly with vegetable oil or melted butter.
Add batter to waffle iron according to manufacturer’s instructions and cook until waffles are golden brown.
Serve immediately, or keep warm in a 200°F oven while you finish cooking. You can place waffles directly on a clean oven rack or place them on a baking rack set into a rimmed baking sheet.