Combine bread flour, whole-wheat flour, milk powder, sugar, salt and yeast in a medium bowl; set aside.
Add ½ cup whole milk, egg and melted butter to stand mixer with Tangzhong; stir to combine.
Add flour mixture to stand mixer, attach dough hook to mixer and turn mixer to low speed. Mix and knead until a smooth, elastic dough forms, 8–9 minutes.
Remove dough from mixer, briefly knead a couple of times by hand then transfer to a large greased bowl.
Cover bowl with a kitchen towel or loosely with plastic and let rest until puffy but not quite doubled in size, 75–90 minutes.
Gently punch down dough.
Divide it into 8 equal pieces.
Form dough pieces into smooth, taut rounds (see recipe note below). Then place rounds in a generously greased 9-inch cake pan (round or square).
Cover Pan with a kitchen towel or loosely with plastic and let rounds rest until puffy, 45–50 minutes.
Meanwhile, heat the oven to 350°F (176ºC) with rack set in middle position.
Using a pastry brush, lightly brush rounds with milk. Bake buns until tops are golden brown and an instant-read thermometer inserted in the center registers 190ºF (87ºC), 25–28 minutes.
Transfer pan to a wire rack; let buns cool in pan for 10–25 minutes. Run a butter knife around edges of pan and gently lift buns out of pan; transfer to wire rack.
Brush tops of buns with melted butter and sprinkle with sea salt.