Preheat your waffle iron according to the manufacturer’s instructions. Lightly grease with butter or nonstick spray.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.
In a separate bowl, whisk the egg yolks, milk, melted butter, and vanilla until smooth.
Pour the wet ingredients into the dry ingredients. Stir gently until combined. The batter should look slightly lumpy, not completely smooth.
In a clean bowl, beat the egg whites with a hand mixer until soft peaks form. Gently fold the beaten egg whites into the batter with a spatula.
Pour about ½ to ¾ cup of batter (depending on your waffle iron size) into the preheated iron. Close and cook until golden brown and crisp, usually 3–5 minutes.
Carefully remove the waffle and keep it warm in a low oven (200°F) while you cook the rest.
Serve hot with your favorite toppings.