PREHEAT the waffle iron according to the manufacturer's instructions. The iron is ready when a few drops of water sprinkled on its surface turn into dancing drops.
SIFT the flour, sugar, baking powder, baking soda, and salt into a large bowl and stir or whisk well.
STIR together the buttermilk, melted butter, egg yolks, and vanilla in a separate bowl.
MAKE a well in the center of the dry ingredients and pour the wet ingredients into it. Stir just until the dry and wet ingredients are combined. Do not over mix.
BEAT the egg whites with a hand-held electric mixer set on medium-high speed. (Be sure the bowl is grease-free.) Beat the egg whites until they start to form stiff peaks when the beaters are lifted.
FOLD one third of the beaten whites into the batter to lighten it. Fold in the remaining whites. Do not over mix or you will deflate the batter you should have white fluffs remaining.
POUR enough batter onto the center of the waffle iron to fill it about two-thirds full. Lower the lid and cook for 3 to 5 minutes or until the waffles are set. You can tell when they are cooked with steam stops seeping out of the sides of the iron.
SERVE immediately or transfer the waffles to a 200° F oven to keep warm while making the remaining waffles.