Place the milk & water into a jug & warm in the microwave to 38°c/100°f (alternatively you could heat this up in a saucepan then transfer to a jug) then whisk in the yeast & leave to go bubbly, 5-10 minutes.
To the bowl of a stand mixer, add the flour, salt, sugar, milk powder, egg, oil & the cooled down tangzhong. Give the activated yeast a whisk then add to the bowl. Using the dough hook, knead the dough on a medium speed until smooth & starting to come away from the sides of the bowl This will take around 5 minutes.
With the mixer still going, add in the butter a small amount at a time, incorporating each piece fully before adding more. Once all the butter has been incorporated, keep mixing until the dough passes the window pane test. This will take another 5-10 minutes.
Transfer the dough to a lightly oiled bowl, cover with clingfilm (or a tea towel) & leave to double in size at room temperature This will take 1-1½ hours.
Once risen, knock the dough back then transfer to a lightly floured work surface. Using a bench scraper, divide the dough into 4 equal pieces, each weighing around 100g.
Shape each piece of dough into a ball (check out my video tutorial on shaping buns) then transfer to a greased & lined baking tray, leaving plenty of room between each bun.Cover with another baking tray (flipped upside down) or loosely with oiled clingfilm & leave to double in size at room temperature, 45 minutes - 1 hour.