Add the bread flour, sugar, salt, and instant yeast to the bowl of a stand mixer and briefly whisk with a fork to combine. Add the egg, warm milk, and cooled water roux starter. Attach the dough hook and mix on speed 1 (low) for 1 minute until the ingredients come together, then increase to speed 3 (medium) and mix for 3 minutes until a shaggy dough forms and begins to pull away from the sides.

With the mixer on speed 3 (medium), add the softened butter a few small pieces at a time, waiting until each addition is mostly absorbed before adding the next. Continue mixing for 5 minutes until the dough is smooth, elastic, and pulls cleanly away from the sides of the bowl. To test, stretch a small piece of dough between your fingers — it should form a thin, translucent membrane without tearing (windowpane test).

Lightly oil the mixing bowl, return the dough to it, and cover tightly with cling film or a damp tea towel. Place in a warm spot — around 28–30°C / 82–86°F — for approximately 1 hour or until doubled in size. If your kitchen is cold, turn your oven on to its lowest setting for 2 minutes, switch it off, then place the covered bowl inside with the door slightly ajar.

Gently punch down the risen dough to deflate it. Turn it out onto a lightly floured surface and divide into 6 equal portions of approximately 60g each. Roll each portion into a smooth ball, place on the work surface seam-side down, cover loosely with a damp tea towel, and rest for 15 minutes to relax the gluten.

Take one rested dough ball and flatten it with your palm or a rolling pin into a round disc approximately 10cm (4 inches) across. Place a heaped tablespoon (about 13g) of pork floss in the centre. Gather the edges of the dough up and over the filling, pinching firmly to seal. Place seam-side down on a lightly floured surface. Repeat with remaining dough portions.

Line a baking tray with parchment paper and transfer the filled buns seam-side down, leaving at least 5cm (2 inches) between each bun to allow for spreading. Cover loosely with a damp tea towel or lightly oiled cling film and leave to proof for 45 minutes at room temperature until the buns are noticeably puffed and spring back slowly when gently pressed.

Gently brush a thin, even layer of beaten egg wash over the top and sides of each proofed bun. Sprinkle white sesame seeds over the tops.

Bake in an oven preheated to 170°C / 340°F (150°C fan-forced) for 15–18 minutes until the tops are evenly golden brown. If your oven has hot spots, rotate the tray halfway through baking.

Remove the buns from the oven and transfer immediately to a wire cooling rack. Allow to cool for at least 10 minutes before serving — the filling will be very hot straight from the oven.