How to Make It
Peel potato, cut into 1-in. chunks, and put in cold salted water. Bring to a boil, covered; then reduce heat to a simmer and cook until very tender, about 20 minutes. Drain, return to pot, and let dry over low heat, stirring, until potato begins to look white and chalky, about 2 minutes. Using a ricer or potato masher, rice or mash potato.
Add warm potato to bowl of a stand mixer along with butter. Using dough hook, mix on low speed until incorporated. Add buttermilk, yeast (crumbled if fresh), egg, sugar, salt, and 1 cup whole-wheat flour. Mix for 3 minutes on low.
Still on low speed, add 1 more cup whole-wheat flour and incorporate before adding bread flour in two batches, letting the first incorporate before adding the next. Mix until dough comes together around hook in a soft, smooth, only slightly sticky ball; if it's sticky, mix in up to 1/2 cup more whole-wheat flour. Mix 6 minutes more.
Lightly flour a large bowl and turn dough into it. Cover with a damp kitchen towel, set in a warm place, and let rise until doubled, about 1 hour.
Turn dough out onto a cornmeal-dusted work surface and sprinkle with more cornmeal. Punch down and cut into 8 equal portions. With wet hands, roll each into a tight, smooth ball, using your thumb to tuck ends under; then sprinkle with a little more cornmeal. Set each ball as finished on a lightly oiled baking sheet, using 2 baking sheets total and spacing the balls as far apart as possible. Cover each with a damp kitchen towel, set in a warm place, and let rise until doubled, about 45 minutes. Meanwhile, preheat oven to 400°.
Bake buns, switching positions of pans halfway through for even baking, until golden brown, 15 to 20 minutes.
*Fresh cake yeast makes the dough rise a little quicker than active dry yeast; find it with the eggs at well-stocked grocery stores.
Arc, Costa Mesa, CA