Preheat waffle iron. In a medium-size bowl, whisk together dry ingredients.
In the bowl of a standing mixer or with a handheld mixer, beat egg whites until soft peaks form. Set aside.
In a third bowl, whisk together egg yolks, milk, and butter. Stir egg yolk mixture into flour mixture until combined (some lumps are fine). Gently fold in egg whites until there are few streaks; then fold in apple.
Spoon into a greased waffle iron and cook until nicely browned; serve immediately with maple syrup.