Preheat waffle iron according to manufacturer's instructions.
In a large bowl, mix together the flour, sugar, baking powder, and salt.
In another bowl, whisk together the mashed sweet potato, milk, egg yolks, melted butter, and vanilla extract until smooth.
Combine the wet ingredients with the dry ingredients, mixing just until moistened.
In a separate bowl, beat the egg whites until stiff peaks form and gently fold them into the waffle batter.
Pour the recommended amount of batter onto the hot waffle iron and cook until the waffles are golden brown and crisp.
Repeat with remaining batter. Serve warm with your favorite toppings.