How to Make It
In a medium saucepan, warm the milk over moderately high heat until bubbles appear around the edge. Remove from the heat and stir in the cornmeal, molasses, chocolate, caraway seeds and salt. Let the mixture cool completely.
Add the egg yolks and yeast to the batter; stir to dissolve the yeast. Stir in the all-purpose flour. Cover loosely with plastic wrap and let stand in a warm place for 1 hour. Stir in the whole wheat and rye flours, then cover and refrigerate overnight.
Bring the waffle batter to room temperature. Preheat a lightly oiled 8-inch-square waffle iron. Stir the mashed potatoes and melted butter into the batter. In a stainless-steel bowl, beat the egg whites with the sugar until soft glossy peaks form. With a rubber spatula, fold one-third of the beaten whites into the batter, then fold in the remaining whites.
Pour a scant 1 1/2 cups of the waffle batter into the preheated iron and bake until browned and crisp, about 7 minutes. Transfer the waffles to a platter and cover loosely with foil. Bake the remaining waffle batter; lightly brush the iron with oil between batches if necessary.