Preheat a waffle iron according to manufacturer’s instructions.
Sift flour, baking powder, and salt together in a bowl. Whisk milk and egg yolks together in a separate bowl. Beat egg whites in a separate bowl using an electric mixer until stiff peaks form. Stir egg yolk mixture into flour mixture just until batter is moistened. Stir butter into batter and fold in egg whites.
Ladle about 1/4 cup batter into preheated waffle iron and cook according to manufacturer’s instructions until golden, about 5 minutes; repeat with remaining batter.
Vegetable oil can be substituted for the melted butter.
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