Heat waffle iron.
Add egg whites to bowl of mixer.
Using whisk attachment, beat egg whites until stiff.
Move beaten egg whites to separate large mixing bowl.
Clean and dry bowl of mixer.
Add dry ingredients (flour, baking powder, salt, and sugar) to bowl of mixer.
Mix together, using paddle attachment at low speed.
In separate bowl, combine milk and egg yolks.
Add milk/egg mixture to dry ingredients.
Mix until just blended, at slow speed.
Add butter and blend again.
By hand, fold egg whites into flour/milk mixture.
Cook waffles per waffle iron instructions, using one ladleful of batter per waffle.