In a large bowl, whisk together flour, baking powder, and salt and set aside.
In another bowl combine the sweet potatoes, milk, brown sugar, butter, and grated orange rind. Stir the sweet potato mixture into the flour mixture and thoroughly combine. Beat egg whites until stiff peaks form. Gradually fold egg whites into batter 1/3 at a time. The batter will be thick. Using a No. 20 disher (scoop), place 2 scoops of batter onto a preheated, oiled waffle iron, and cook until lightly browned, about 5 to 6 minutes.
Per waffle: Calories: 465; Total Fat: 13 grams; Saturated Fat: 8 grams; Protein: 14 grams; Total carbohydrates: 73 grams; Sugar: 21 grams Fiber: 2.5 grams; Cholesterol: 36 milligrams; Sodium: 841 milligrams