• Put the potato chunks in a saucepan, cover with water, and bring to a boil over high heat. Reduce the heat to medium-low and cook for about 15 minutes, or until the potato chunks are fork-tender. Drain and mash the potatoes and set aside to cool.
  1 medium russet potato
• In a large bowl or the bowl of a stand mixer, combine the yeast and warm water. Let stand until foamy, about 10 minutes.
  2 1/4 tsp active dry yeast (1 packet) | 1/2 cup warm water
• Add the lightly beaten egg, milk, softened butter, and 3/4 cup of the cooled mashed potato to the yeast mixture. Blend well. Add the sugar, salt, and about half of the flour. Blend well by hand or with the paddle attachment of a stand mixer.
  1 large egg | 2/3 cup milk | 4 tbsp softened butter | 1 1/2 tbsp granulated sugar | 1 1/4 tsp fine salt | 3 1/2 cups (482 grams) bread flour
• If using a stand mixer, change to a dough hook. Add the remaining flour and knead with the dough hook for about 8 minutes, or knead by hand on a floured surface for about 10 minutes. Add a little more flour if necessary to keep the dough from sticking to hands, surface, or the sides of the bowl.
• Oil or butter a large bowl. Gather the dough into a ball and place it in the bowl. Turn over a few times to grease all sides of the dough. Cover with plastic wrap and let the dough rise in a draft-free place for about 45 minutes to 1 hour, or until the dough has doubled in bulk.
  Vegetable oil or butter for greasing the bowl
• Line 2 large baking sheets with parchment paper.
• On a floured surface, punch the dough down and pat it into a rectangle. Cut the dough into 16 to 18 pieces, roughly 2 ounces each. Shape into taut balls, flatten slightly with your hand, and place them on the parchment paper-lined baking sheets, leaving at least 2 1/2 inches between them.
• Cover the buns with a lightweight kitchen towel and let them rise in a draft-free place for about 30 minutes.
• Meanwhile, preheat the oven to 375° F (190° C/Gas 5).

Make the Egg Wash and Bake the Buns

Tip: The egg wash adds shine and helps the seeds adhere, but you can skip it and brush the buns with melted butter as soon as the buns come out of the oven.
• Combine the egg with 1 tablespoon of water. Whisk to blend. Just before the buns go into the oven, brush them gently with the egg wash mixture. Sprinkle the tops with sesame seeds or poppy seeds, if desired.
  1 large egg | 1 tbsp water | Sesame seeds or poppy seeds
• Bake the buns for 15 to 18 minutes, or until golden brown.
  
• Makes about 35 ounces of dough, enough for 16 to 18 2-ounce buns.