Instructions

• Preheat the oven to 400°F. Rinse and dry the russet potato, pierce several times with a fork, place on a parchment-lined baking sheet, and bake 60–70 minutes, until tender when pierced.
• While the potato bakes, prepare the tangzhong: measure 2 tablespoons of the all-purpose flour (from the 4 cups), 1/4 cup of the whole milk, and 1/4 cup of the water. Whisk these three together in a small saucepan until smooth.
• Cook the tangzhong over low heat, whisking constantly, until it becomes a thick paste (about 7–10 minutes). Transfer to a small bowl and let it cool to room temperature.
• From the 3/4 cup whole milk, measure and set aside 2 tablespoons for the final wash. Warm the remaining milk (the rest of the 3/4 cup after removing the 1/4 cup used in the tangzhong and the 2 tablespoons reserved for the wash) to 112–115°F. Warm the remaining water (the 1/2 cup total minus the 1/4 cup used in the tangzhong = 1/4 cup) to 112–115°F as well.
• In a small bowl, combine the warm milk and warm water, add 2 tablespoons of the sugar (from the 1/3 cup), and sprinkle in the active yeast. Whisk and let sit 5–7 minutes until foamy (a raft forms).
• In a separate large bowl, whisk together the remaining all-purpose flour (the 4 cups minus the 2 tablespoons reserved for tangzhong), the remaining sugar (the rest of the 1/3 cup after the 2 tablespoons used for the yeast), and the 1 1/2 teaspoons coarse salt until evenly mixed.
• Once the yeast mixture is foamy, transfer it to the bowl of a stand mixer fitted with the dough hook. Add the riced/bashed potato (see next step), then slowly add the flour mixture while mixing on low. Immediately add the whole egg and the cooled tangzhong, and continue mixing on low to medium until the dough comes together.
• Prepare the baked potato: remove it from the oven, let it rest 5–7 minutes until cool enough to handle, peel it, and pass it through a ricer or food mill (or mash very well) so it is smooth. Add the mashed potato to the mixer as directed in step 7.
• When the dough begins to pull away from the bowl and cling to the hook, add the 1/3 cup softened unsalted butter in three additions, waiting until each addition is fully incorporated before adding the next. Mix until the dough mostly clears the sides of the bowl and is smooth and elastic.
• Transfer the dough to a lightly oiled airtight container or a lightly oiled large bowl covered with plastic wrap or a lid. Let rise in a warm place about 1 hour, or until doubled in size.
• Punch down the dough and divide it into 10 equal pieces (about a heaping 1/2 cup packed or ~125 g each). Shape each piece into a tight ball by folding edges into the center and rolling on the counter to create a smooth, taut top.
• Place 5 dough balls on a parchment-lined baking sheet (space them evenly). Cover with a clean kitchen towel and let rise 1 hour, or until noticeably puffy and nearly doubled. Repeat with the remaining 5 balls on a second sheet.
• Place a small oven-safe pan on the bottom rack of the oven (you will add hot water to this pan to create steam). Preheat the oven to 375°F (use convection if available). When the oven is preheated, carefully pour hot tap water into the pan on the bottom rack to generate steam.
• Make the wash: whisk the 2 tablespoons of milk you set aside earlier and brush it lightly over the tops of the risen buns.
• Bake the buns at 375°F for 17–20 minutes, until golden brown and the internal temperature reaches about 208°F.
• Remove the buns from the oven and transfer to a cooling rack. Let cool to room temperature before serving or storing.