Place the cup of warm water in a large bowl, and sprinkle the yeast over. Swish it around, and let it stand for about 5 minutes. Add one cup of the white flour, and whisk until you have a smooth batter. This is the sponge. Cover and let it rise in a warmish place for 4 - 8 hours. At this point you can either boil the potato and mash it, OR get out the leftover mashed potatoes, buttermilk, and butter, and let them come to room temperature while the sponge rises. Add all the remaining ingredients except the flour, then add the flour a cup at a time, mixing until the dough pulls away from the bowl. Turn the dough out onto a floured surface and knead until it is smooth. Form the dough into a ball, and place it back in the scraped out bread bowl, cover and let rise until doubled, about 1 to 1 1/2 hours. Punch the dough down, turn it onto the floured surface again, and knead just a bit to redistribute the yeast. Divide the dough into roll-size chunks, I think there will be about 18. I like to roll the dough into logs, and tie them in knots for shaping, but you can use whatever roll shape you like. Arrange the rolls on greased or sprayed baking sheets, leaving about an inch or two between to allow them to rise. Brush with egg wash (an egg mixed with a little water or milk) and sprinkle with black sesame seeds, or other seedy topping of your choice. Preheat the oven to 400, and let the rolls rise about another 45 minutes. Bake 20 - 30 minutes until nicely browned.
Portrait of Ginerva de' Benci, 1474, by Leonardo da Vinci
National Gallery of Art, Washington D.C.