Place sweet potatoes in a steamer basket and place the basket in a large pot of simmering water. The water should beat least 2 inches from the bottom of the basket. Cover and steam over medium-high heat until the sweet potatoes are fork-tender, about 20 minutes.
Remove steamer basket, pour the water out of the pot, and dump the sweet potatoes into the still-hot pot. Mash with a potato masher until mostly smooth.
In a large mixing bowl, combine the sweet potatoes, milk, brown sugar, butter, and orange zest with a wooden spoon.
Sift together the flour, baking powder, and salt, and add to the sweet potato mixture. Stir to incorporate, but do not overmix; there should be lumps remaining in the batter.
In a separate bowl, using an electric hand mixer, beat the egg whites until stiff peaks form. Fold the egg whites into the batter one third at a time. The batter will be thick. Using a disher (appropriate to your waffle iron's capacity), dose the batter onto a heated, oiled waffle iron, and cook until lightly browned, 5 to 6 minutes. Serve hot, if you know what's good for you.