Preheat the waffle iron for at least 10 minutes before using. Whisk together flour, Clabber Girl Baking Powder, sugar and salt in a large bowl. Melt butter and set aside to cool until tepid (body temperature). Combine milk and egg yolks in a small bowl and whisk until well-blended. Stir in the cooled butter. Make a well in the center of the flour mixture and pour in the milk mixture. Using a large spoon, stir until combined. Do not over mix. In a small bowl with an electric mixer, whip egg whites until soft peaks form. Fold whipped egg whites into batter until no white streaks remain Pour about 1/2 cup batter over the bottom of each hot waffle grid. Close lid and bake until golden (when almost all the steam stops flowing out of the iron) about three to four minutes. For best flavor and a crispy texture, serve waffles hot. (Waffles lose crispiness as they cool). To keep warm while preparing additional waffles, set waffles on a baking sheet and place in a preheated 225 F oven. If you stack them, they are apt to soften.