In a small bowl, combine the warmed milk, sugar, and yeast. Give it a stir and let the yeast activate and become bubbly, a few minutes.
In a stand mixer bowl fitted with a dough hook attachment, add in the cooled tangzhong mixture, flour, sea salt, egg, oil, and the activated yeast mixture.
Knead until the dough becomes shiny and smooth, at least 5-8 minutes. The dough may appear wet at first, but it will come together.
Transfer the dough to a lightly greased bowl and cover with a tea towel in a warm location. Leave it to rise until doubled in size, about 1-1.5 hours, depending on how warm your kitchen is.
Deflate the dough and divide into 8 equal portions, about 76g each.
Shape the dough into a round ball with your hand and place onto a large parchment-lined baking sheet. (I used a 9" by 17" baking sheet).
Allow some space between each dough for it to expand. Repeat with the remainder.
Cover the dough loosely with a tea towel and let it become slightly puffy, about 20-30 minutes.
Preheat oven to 350°F/177°C.
Remove the butter topping from the fridge.
Use a sharp pairing knife to make parallel slits on each topping going in one direction, and then repeat going in the opposite direction, so that you end up with "hashtags," or the visual effect of a pineapple.
Use a dough scraper to transfer the butter topping onto the surface of each dough ball.
Use a pastry brush to lightly brush the topping with egg white (or milk).
Bake at 350°F/177°C for 18-20 minutes, until lightly golden in colour.
Remove from the oven and let cool slightly on a wire rack.