In a large bowl, whisk together the flour, sugar, baking powder, baking soda and salt. Set aside. In a smaller bowl, whisk together 6 tablespoons of the butter, buttermilk, milk and egg yolks. Pour the wet ingredients into the dry and stir until combined. In a large metal or glass bowl, beat the egg whites until nearly stiff (don’t stress over this too much. You aren’t frosting a cake. Get as close to stiff peaks as your arm will allow). Fold the egg whites into the batter until fairly well incorporated.
Use the remaining butter to grease the waffle iron. Pour enough batter to fill your iron (mine takes about 2/3 cup per waffland cook until the outside of the waffle is brown and crisp. Serve immediately or hold in a warm oven until ready to serve. Continue with the remaining batter.