Instructions

for the flour paste

Whisk ⅔ cup whole milk and ¼ cup (1¼ ounces) flour in small bowl until no lumps remain. Microwave, whisking every 25 seconds, until mixture thickens to stiff paste, 50 to 75 seconds. Whisk until smooth.

for the dough

In bowl of stand mixer, whisk flour paste and ⅔ cup whole milk until smooth. Add 1 large egg and 1 large yolk and whisk until incorporated. Add 3¼ cups (16¼ ounces) flour and 2¼ teaspoons instant yeast. Fit mixer with dough hook and mix on low speed until mass of dough forms and all flour is moistened, 1 to 2 minutes. Turn off mixer, cover bowl with kitchen towel or plastic wrap, and let dough stand for 15 minutes.
Add 3 tablespoons sugar and 1½ teaspoon table salt to dough. Knead on medium-low speed for 5 minutes. Add 6 tablespoons softened unsalted butter cut into 6 pieces and continue to knead until incorporated, scraping down dough hook and bowl as needed (dough will be sticky), about 5 minutes longer.
Transfer dough to lightly floured counter and knead briefly to form ball. Transfer, seam side down, to greased large bowl. Cover tightly with plastic and let rise at room temperature until doubled in size, about 2 hours.

for the filling

Meanwhile, combine 9 chopped scallions and 8 tablespoons softened butter in small bowl, mixing with fork. Spray 13 by 9-inch metal baking pan with vegetable oil spray
Once dough has sufficiently risen, use dough scraper to transfer dough to lightly floured counter. Use either floured rolling pin or your hands to stretch dough to 18 by 15-inch rectangle. Use offset spatula to spread butter-scallion mixture over entire surface of dough, leaving 1-inch border. Sprinkle 3 cups shredded extra-sharp cheddar in even layer and gently press into dough. Drizzle ½ cup chili crisp over cheese.
Beginning with long edge, roll dough into even log, pushing in ends to create even thickness. Pinch seam to seal, and slice into 12 equal buns. Place buns, cut side down, in prepared pan in 3 rows of four. Cover tightly with plastic and let rise until buns are puffy and touching one another, about 1 hour.
Adjust oven rack to middle position and heat oven to 375 degrees. Discard plastic and bake buns until golden brown on top, about 35 minutes. Let cool for 10 minutes. Drizzle with extra chili crisp, sprinkle with extra sliced scallions, and serve.