Instructions

for the flour paste

Whisk ⅔ cup water and ¼ cup (1½ ounces) bread flour in small bowl until no lumps remain. Microwave, whisking every 25 seconds, until mixture thickens to stiff, pasty consistency, 50 to 75 seconds.
In bowl of stand mixer, whisk flour paste and ⅔ cup milk, chilled, until smooth. Whisk in 1 large egg. Add 2¾ cups (15¼ ounces) bread flour and 2 teaspoons instant or rapid-rise yeast. Fit stand mixer with dough hook and mix on low speed until all flour is moistened, 1 to 2 minutes. Let stand for 15 minutes.
Add 2 tablespoons sugar and 1½ teaspoons table salt and mix on medium-low speed for 5 minutes. Scrape down sides of bowl. Add 4 tablespoons softened unsalted butter and continue to mix on medium-low speed for 5 minutes longer, scraping down dough hook and sides of bowl halfway through (dough will stick to bottom of bowl).
Transfer dough to lightly oiled counter. Knead briefly to form ball and transfer seam side down to lightly greased bowl. Cover dough and let rise until just doubled in volume, about 1 hour.
Grease 13 by 9-inch baking pan. Transfer dough to lightly oiled counter. Press dough gently but firmly to expel all air. Pat and stretch dough into 4 by 10-inch rectangle with long side parallel to counter edge. Using knife or bench scraper, halve dough to create 2 long strips. Divide each strip into 5 equal portions (88 grams or just over 3 ounces each). Press each portion into 5-inch square, roll into 6-inch cylinder, and pinch seam. Arrange cylinders seam side down in 2 rows of five in prepared pan. Cover and let rise until doubled in size, about 1½ hours.
Twenty minutes before baking, adjust oven rack to middle position and heat oven to 350 degrees. Bake until buns are golden brown and register at least 200 degrees, 20 to 25 minutes. Let buns cool in pan on wire rack for 3 minutes; invert buns onto rack, then reinvert. Let cool completely, at least 1 hour.
Once buns have cooled, use serrated knife to trim crust from each long side of attached buns, then slice buns apart at seams to create 10 buns. Slice each bun lengthwise, stopping three-quarters of way from bottom.