Preheat your waffle iron according to the manufacturer’s instructions.

In a large bowl, whisk together the flour, baking powder, salt, and sugar.

In another bowl, whisk the egg yolks, then add milk, melted butter, and vanilla extract. Mix until combined.

Pour the wet ingredients into the dry ingredients and stir until just blended; do not overmix.

In a clean bowl, beat the egg whites until they form stiff peaks.

Gently fold the whipped egg whites into the batter. Do not overmix; it's okay if there are a few streaks.

Lightly grease the waffle iron with butter or non-stick spray.

Pour enough batter into the waffle iron to just cover the waffle grid (about ½ to ¾ cup per waffle, depending on your iron). Close the lid and cook until golden brown and crisp (usually 4–5 minutes).

Remove the waffle and repeat with the remaining batter.