Preheat your waffle iron to medium-high and grease grids with non-stick spray or melted butter.
In a large bowl, whisk together flour, sugar, baking powder, and salt until evenly combined.
Separate eggs, placing yolks in a medium bowl and whites in a clean grease-free bowl. Set both aside.
To the yolks, add milk, melted butter, and vanilla. Whisk until smooth and fully combined.
Pour wet mixture into dry ingredients and stir with a spatula until just combined. A few small lumps are fine. Do not overmix.
Beat egg whites with a hand or stand mixer on medium until foamy, then increase to high and beat to stiff peaks, about 2-3 minutes.
Gently fold half the beaten whites into the batter with a rubber spatula using a scooping motion. Fold in the second half until just combined; some streaks are fine.
Pour about 3/4 cup batter into the center of the preheated waffle iron, close the lid, and cook for 4-5 minutes until steam slows and waffle is golden brown.
Transfer finished waffles to a wire rack placed on a baking sheet and keep warm in a 200°F oven while cooking remaining batches.
Serve immediately with your favorite toppings like maple syrup, berries, whipped cream, or butter.