Preheat a waffle iron to medium-high.

In a large bowl, whisk together the flour, baking powder, salt, and nutmeg.

In a medium bowl, whisk together the milk, egg yolks, vanilla, and 1/4 cup melted butter. Whisk the milk mixture into the flour mixture until a batter is formed. Take care not to over mix the batter.

In medium bowl, with a hand-held electric mixer or whisk, whip the 3 egg whites until they just begin to hold a loose peak. Scatter the sugar over the whites and continue beating until they hold a soft peak. Using a rubber spatula, fold a third of the egg whites into the batter to lighten the base. Fold in the remaining whites.

Brush the inside surface of the iron with butter. Pour in enough batter to lightly cover the surface of the iron, about 1/3 to 3/4 cup, depending upon the size of the iron. (Take into consideration that the batter will spread once the lid is closed.) Cover and cook until golden brown and slightly crisp, about 5 minutes. Repeat with the remaining batter. (If the waffle iron is well seasoned or nonstick it is not necessary to brush the inner surface each time before cooking the waffles.)

Serve the waffles immediately with syrup, jam, fresh fruit, or yogurt.