In the bowl of a stand mixer fitted with a dough hook, combine all the ingredients listed for the main dough, excluding the butter, including the tangzhong.
360 g bread flour, 50 g sugar, 7 g instant dry yeast, 5 g salt, 1 large egg, 130 g whole milk, All the tangzhong
Knead the dough on low to medium speed for about 10 minutes.
Gradually add the cubed butter to the dough, one cube at a time, while continuing to knead.
85 g unsalted butter
Continue kneading for another 10-20 minutes or until the dough passes the window-pane test - it should be smooth, elastic, and able to be stretched thinly without tearing.
Transfer the dough to a clean, lightly oiled bowl.
Cover the bowl with plastic wrap and let the dough proof for about 90 minutes, ideally at around 77 °F (25°C), until it doubles in size.