Vigorously whisk together 3 tablespoons water, 3 tablespoons whole milk, and 2 tablespoons bread flour in a small saucepan until thoroughly combined and no lumps remain.

Cook Tangzhong over low heat, whisking constantly, until thickened and whisk leaves a trail on bottom of pan, 3–5 minutes.

Transfer Tangzhong to the bowl of a stand mixer (or large bowl is mixing by hand) and let cool to room temperature.

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.