Heat waffle iron while mixing batter.
In a medium bowl, whisk together flour, sugar, baking powder, baking soda and salt. Set aside.
In a separate medium bowl, whisk together buttermilk, melted butter and egg yolks. Add to the dry ingredients and stir until combined. Batter will be on the thicker side, but will fluff up after adding egg whites. If the batter is extremely thick, add a bit more buttermilk to thin it out.
In a small bowl, using an electric mixer, beat egg whites until soft peaks form, about 1-2 minutes.
Gently fold the egg whites into the batter until just combined.
Spray waffle iron with nonstick spray. Pour batter into waffle iron and cook according to your waffle iron specifications.
Serve hot with favorite toppings.