Closely related to potato pancakes, the potato waffle has one advantage -- it reheats exceptionally well and therefore can be made in advance. Serve with your favorite brunch foods, such as soft-boiled eggs and poached asparagus.
Peel potatoes, and cut each into eighths. Place in a medium saucepan of cold salted water. Bring to a boil, and simmer until potatoes are very tender, about 8 minutes. Drain, and set aside in a bowl to cool. Mash into coarse pieces with a fork or pastry cutter.
Heat the waffle iron. In a large bowl, sift together all purpose flour, sugar, baking soda, baking powder, and salt. Set aside.
In a medium bowl, whisk together egg yolks, buttermilk, and butter. Pour into the dry ingredients, and stir briefly, until just combined. Stir in potatoes.
In another medium bowl, beat egg whites until stiff but not dry. Fold into the batter.
Ladle about 1/3 cup batter onto each section of the waffle grid; spread batter almost to the edges. Close lid; bake until no steam emerges from waffle iron, 5 to 6 minutes.
Transfer waffles to a baking sheet; place in an oven set to low heat, about 200 degrees, while using remaining batter. Serve warm.