Soften the butter at room temperature until you can easily press it with your finger. Chop the scallions and set aside. Line a baking tray with parchment paper.

In a large bowl, add the milk, egg, salt, sugar, and instant yeast. Stir thoroughly with chopsticks until the sugar and salt are fully dissolved.

Slowly pour the liquid mixture into the bread flour.

Stir until a shaggy dough forms with no dry flour remaining.

Add the softened butter.

Wearing gloves if desired, knead gently until the butter is fully incorporated and the dough comes together. At this stage, the dough does not need to be smooth.

Cover the bowl tightly with plastic wrap and place it in the refrigerator for 20 minutes to relax the gluten.

Remove the dough from the refrigerator. Using a stretch-and-fold motion — pulling the edges toward the center — knead until the dough becomes smooth, fine, and no longer sticky.

Cover the dough and let it ferment in a warm place (28–30°C / 82–86°F) until doubled in size, about 40–60 minutes.

To check readiness, lightly flour your finger and poke the dough. If the indentation does not shrink back or collapse, the dough is ready.

Gently press the dough with your palm to release excess gas.

Divide the dough evenly into 8 portions and roll each into a round ball.

Take one portion and roll it into an oval shape.

Slightly flatten one end to help with sealing later.

Pipe an appropriate amount of mayonnaise onto the end of the dough that is farther away from you, then spread an even layer of pork floss on top.

Starting from the unflattened end, roll the dough tightly toward the flattened end. Pinch the seam firmly to seal and shape it into an olive form.

Place on the prepared baking tray. Repeat with the remaining dough.

Place the tray in the oven with the oven turned off. Put a bowl of warm water at the bottom of the oven to create humidity.

Let the buns rise until they reach about 1.5 times their original size, approximately 20–30 minutes. The dough should feel light and airy.

Remove the tray and brush the surface of the buns evenly with egg wash.

Preheat the oven to 160°C / 320°F (top and bottom heat) for about 10 minutes.

Pipe mayonnaise and ketchup on top, then sprinkle with chopped scallions.

Bake on the middle rack at 160°C for 25 minutes, until the buns are golden brown and spring back slightly when pressed.

Remove from the oven and let cool for 5 minutes before serving.