In a mixing bowl, sift together the flour, sugar, baking powder, and salt. In a separate bowl, beat the egg yolks. Add the buttermilk, milk, and melted butter. Add the sifted dry ingredients to the bowl with the eggs and milk. In a small bowl, beat the egg whites until thick and foamy. Gently add them to the batter, and mix to combine. Pour the waffle batter into a hot waffle iron, and cook until golden brown and lightly crisp.