In a glass measuring cup, combine 1 cup warm water, the milk, yeast and sugar. Let stand until foamy, about 5 minutes. Meanwhile, beat 1 egg.

In a large bowl, whisk flours with salt. Add butter and rub into flour between your fingers, making crumbs. Using a dough scraper, stir in yeast mixture and beaten egg until a dough forms. Scrape dough onto clean, unfloured counter and knead, scooping dough up, slapping it on counter and turning it, until smooth and elastic, 8 to 10 minutes.

Shape dough into a ball and return it to bowl. Cover bowl with plastic wrap and let rise in a warm place until doubled in bulk, 1 to 2 hours.

Line two baking sheets with parchment paper. Using dough scraper, divide dough into 8 equal parts. Gently roll each into a ball. Arrange 4 of the dough balls onto each baking sheet, 2 to 3 inches apart. Cover loosely with a clean kitchen towel and let buns rise in a warm place for 1 to 2 hours.

Set a large shallow pan of water on oven floor. Preheat oven to 400 degrees with rack in center.

If the buns have risen too much and are really flat, reshape into balls.

Beat remaining egg with 1 tablespoon water and brush some on top of buns. Add sesame seeds if using. Bake, turning sheet halfway through baking, until tops are golden brown, about 15 minutes. Transfer to a rack to cool completely.