In the bowl of a stand mixer, whip 3 egg whites until stiff peaks form. Set aside.
Combine dry ingredients in a large mixing bowl and whisk together.
Combine milk, pumpkin, butter, egg yolk and vinegar in a small bowl and stir to combine.
Add pumpkin mixture to dry ingredients and stir to combine.
Gently fold in egg whites.
Use batter immediately in waffle maker.
