Separate the eggs; place the yolks in a large bowl and the whites in a small bowl.
Beat the egg whites until stiff; set aside.
In a medium bowl, sift together the flour, sugar, baking powder and salt.
Alternately add the flour mixture and milk to the egg yolks, mixing well. Add the melted butter and mix again. Finally, fold in the stiffly-beaten egg whites.
Cook waffles in a lightly oiled waffle iron for 3-5 minutes each, or until cooked through and golden brown.
Notes
Beat the egg whites until stiff, and then be very gentle when folding them into your batter. These light and fluffy egg whites will make your waffles light and fluffy on the inside!
Don't over-mix the batter. Stir just until the ingredients are combined. You will still have some lumps, which is fine. Over-mixing will result in dense, tough, and dry waffles.
Use a good waffle iron that will cook your waffles to an even golden-brown color with those beautiful crispy edges. I love my All-Clad waffle iron, but there are many good options on the market.
Make sure to use a brush to lightly oil the waffle iron before adding each round of batter or spray with cooking spray. This will prevent the waffles from sticking to your waffle maker. Just don't use too much oil, or the waffles can come out greasy.
Keep the waffles warm while you finish with the rest of your batter by placing the cooked waffles on a baking sheet in a 200 degree F oven.