Step 1
Whisk ⅔ cup entire milk and 3 Tbsp. (23 g) all-purpose flour in a small saucepan to mix. Place over medium warmth and cook dinner, whisking always, till thickened and pasty, 2–3 minutes. Switch combination (that is your tangzhong) to the bowl of a stand mixer and fasten dough hook. Add ¼ cup (50 g) sugar, one ¼-oz. envelope instantaneous yeast (about 2¼ tsp.), 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, 2 giant eggs, ¾ cup entire milk, and remaining 4 cups (500 g) all-purpose flour and blend on low pace till a shaggy dough kinds, about 1 minute. Add ½ cup (1 stick) room-temperature butter, minimize into items, a bit at a time, beating to include after every addition earlier than including extra. When the entire butter has been added, enhance pace to medium-high and beat till dough is easy and supple, about 10 minutes.
Step 2
Switch dough to a big bowl calmly coated with vegetable oil; flip to coat. Cowl with plastic wrap and let sit in a heat, dry spot till doubled in measurement, about 2 hours. (Or, place in fridge and chill till doubled in measurement, no less than 8 hours and as much as 12 hours).
Step 3
Brush a 13×9″ baking dish effectively with a few of 6 Tbsp. unsalted butter, melted, barely cooled. Punch down dough a few instances to launch air. Portion equally into 24 balls (about 45 g every). Cup your hand over a ball and drag throughout floor, transferring your hand in a round movement, to create a easy, tight ball. Flip over so easy aspect is going through down. Press out to a 4×2″ rectangle and brush with melted butter. Fold in half and place in ready baking dish. Repeat with remaining balls, ensuring to rearrange buns so folded edges are all going through the identical route. Cowl with a kitchen towel; let sit till barely puffed, 45–60 minutes.
Step 4
Place a rack in decrease third of oven and preheat to 350°. Beat remaining 1 giant egg and 1 Tbsp. entire milk in a small bowl to mix. Brush over buns, then sprinkle with flaky sea salt. Bake, rotating entrance to again midway by, till golden brown and tops are crisp to the contact, 35–38 minutes.