Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. Combine all of the dough ingredients, and mix and knead — by hand, mixer, or bread machine set on the dough cycle — until the dough is smooth and satiny., Put the dough in a greased bowl, cover the bowl with plastic wrap, and let the dough rise in a warm, draft-free spot until it's doubled, about 90 minutes to 2 hours., Divide the dough into 10 pieces; if you have a scale, they'll weigh about 2 1/2 ounces (71g) each. Shape each piece into a rough 3" log, and let the logs rest, covered, for about 10 minutes., Working with one piece of dough at a time, flatten it, and fold it in half lengthwise, sealing the seam. Repeat: flatten, fold, and seal. By this time the log will have elongated a bit; flatten it one more time, making a 6" oval that's as even as you can get it., Lay the bun on a lightly greased or parchment-lined baking sheet. Repeat with the remaining pieces of dough, laying the buns about 3/4" from one another, for soft-sided buns; or farther apart for buns with crust all around., Cover the buns lightly with greased plastic wrap, and let them rise until they're noticeably puffy but not doubled, about 1 hour., Preheat the oven to 350°F., Mix the reserved egg white with 2 teaspoons cold water, and brush the mixture over the top of the risen buns. Sprinkle heavily with poppy seeds., Bake the buns for about 18 minutes, until they're golden brown. They may seem slightly "damp." That's OK; they'll dry as they cool., Remove the buns from the oven, and place them on a rack to cool., Store the buns in a plastic bag for a few days on the counter, or store for up to 3 months.