Soft Potato Burger Buns

Method

1. In a large bowl or stand mixer bowl, stir the warm milk and sugar together, sprinkle the yeast on top, and let it sit undisturbed for 8 to 10 minutes until foamy on top; if it does not foam, the yeast is dead and you need a fresh packet.
2. Whisk in the mashed potato, egg, and melted butter until combined and smooth.
3. Add the flour and salt, and mix with a wooden spoon or the dough hook on low speed until a shaggy dough forms.
4. Knead the dough for 8 minutes by hand on a lightly floured counter, or 6 minutes with a stand mixer dough hook on medium-low, until the dough is smooth, elastic, and only slightly tacky (it should not stick heavily to your hands after kneading).
5. Shape the dough into a ball, place it in a lightly greased bowl, cover with plastic wrap or a damp towel, and let it rise in a warm spot for 60 to 90 minutes, until doubled in size (press a finger into the dough; if the indent stays, it is ready).
6. Punch the dough down gently to release air, turn it onto the counter, and divide it into 8 equal pieces, about 3.5 ounces each; a kitchen scale helps but is not required.
7. Shape each piece into a smooth, tight ball by cupping it under your palm and rolling in small circles on the counter, then place the balls on a parchment-lined baking sheet, spaced 2 inches apart.
8. Cover loosely with a towel and let the buns rise again for 45 to 60 minutes, until noticeably puffy and about doubled.
9. Preheat the oven to 375 degrees F during the final 15 minutes of rising.
10. Brush the tops of the buns gently with the egg wash and sprinkle with sesame seeds if using.
11. Bake for 16 to 18 minutes, until the tops are deep golden brown and the buns sound hollow when tapped on the bottom.
12. Let the buns cool completely on a wire rack, at least 30 minutes, before slicing, or the soft crumb will squash when cut.