Preheat waffle iron.

Sift together flour, baking powder, salt, and sugar in a bowl.

In a separate bowl whisk milk, egg yolks and vanilla. Pour over dry ingredients and very gently stir until halfway combined. Pour in melted butter and continue mixing gently until combined.

In a separate bowl using a mixer beat egg whites until stiff. Carefully fold into batter stopping short of mixing them all the way in.

Scoop onto hot waffle iron following iron directions. Serve immediately.