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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.
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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.
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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.
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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.
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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.
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Cover the buns lightly with greased plastic wrap, and let them rise until they're noticeably puffy but not doubled, about 1 hour.
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Preheat the oven to 350°F.
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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.
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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.
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Remove the buns from the oven, and place them on a rack to cool.
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Store the buns in a plastic bag for a few days on the counter, or store for up to 3 months.
