Combine egg yolks, milk, melted butter, and vanilla extract in a small bowl and set aside.
Beat the egg whites in a medium bowl with a mixer on medium-high speed until stiff peaks form.
Mix the egg yolk mixture into the dry ingredients until just combined, then gently fold in the beaten egg whites.
Spoon the batter onto the preheated and greased waffle iron, covering most of the wells, and cook for 3-5 minutes.