Put tangzhong ingredients into a pan and whisk to combine. Place on a low-medium heat and whisk continually until resembling a thick paste. Remove from heat and continue to whisk for a minute or so.
Place tangzhong and milk into stand mixer and mix with whisk attachment. The resulting mixture should be lukewarm.
Add the sugar and yeast and mix again.
Switch to dough hook attachment.
Add the flour, milk powder, salt and egg, and set mixer to a medium slow speed. Knead in mixer for 15 minutes.
After 15 minutes the kneading should have created a very soft dough. Transfer to a floured kitchen work surface and dust with flour, and bring dough into a ball.
Lightly brush the inside of a large glass bowl with sunflower oil, and transfer dough to bowl. Cover with clingfilm.
Place in warmed airing cupboard for 90 minutes.
After dough has doubled in size, turn onto a floured work surface and knock the dough back.
Cut into 8 equal sized pieces, and shape into small round buns by tucking dough edges under and pinching so the buns are rounded on the top.
Rest the dough for ten minutes.
Gently re-roll buns to firm them up, and place on two lined baking trays.
Cover lightly with clingfilm and leave for a second rise of 90 minutes, until doubled in size. Set timer for 75 minutes
Once timer goes off, preheat oven to 185℃ and prepare egg wash by mixing together the egg and water.
Brush buns with egg wash. Sprinkle with sesame seeds if desired.
Bake for 15-18 minutes, until risen and golden brown on top.