Mix the rest of the ingredients in a bowl. Set aside around ½ cup of the mixture for basting. Toss in the pork and coat evenly. Cover and refrigerate for 6 hours or overnight.
Preheat the oven to 350°C (475°F). Line a baking sheet with aluminum foil and place a roasting rack on top. Arrange pork slices and roast for 30 minutes, basting both sides of the pork with the marinade every 10 minutes. Flip the pieces over at every interval. Remove from the oven. Let the meat rest for 10 minutes before using as filling.
Pork Bun Filling
To make the flavouring sauce, combine the sugar, salt, white pepper, soy sauce, oyster sauce, and water in a small bowl. Stir to dissolve the sugar and set aside.
Heat the oil in a medium skillet over medium heat and saute the scallions. Add the pork and the flavouring sauce. Allow to boil until the pork is heated through. Pour the rice wine and dissolved cornstarch to the pork mixture to thicken. Cook for another 30 seconds, stirring constantly, until the mixture comes together.
Transfer to a bowl and set aside to cool completely before using. (The filling can be prepared up to 2 days in advance, covered with plastic wrap, and refrigerated.)
Melt the butter with the milk in a saucepan over medium heat. Set aside to cool.
Put the yeast in small bowl, add the water, and set aside for 1 minute to soften. Whisk in the milk mixture and the egg to blend.
Combine the sugar and flour in a large bowl. Make a well in the centre and pour in the yeast mixture. Slowly stir with a wooden spoon to work in all the flour. Gather and pat the dough into a ball. Transfer to a work surface and knead for about 5 minutes, or until smooth, fingertip soft, and slightly elastic. Press your finger into the dough; it should spring back, with a faint indentation remaining.
Lightly oil a clean bowl and add the dough. Cover with plastic wrap and put in a warm, draft-free place (for example, the oven with the light on) to rise for about 45 minutes, or until nearly doubled.
Line the 2 baking sheets with parchment paper before assembling the buns.
Remove the dough from the bowl and put on a lightly floured surface. Knead for a few times, then cut in half. Cover one-half with plastic wrap or with an inverted bowl to prevent from drying. Roll out the dough into a 12-inch log, and cut into 8 or 16 pieces for medium or small buns, respectively.
Lightly roll each piece between your hands into a ball and flatten each one into a 1/4-inch-thick disc. Use a rolling pin to roll the pieces into discs, about 2 1/2 inches in diameter for small or 3 1/4 inches in diameter for medium buns. The rim should be thinner than the centre. Lay the finished discs out on the lightly-floured work surface.
To form a bun, hold a dough circle in a slightly cupped hand. Use a spoon or fork to centre about 2 teaspoons of filling for small buns, or about 4 teaspoons of filling for medium ones, on the dough circle, pressing down very gently and keeping about 1/2 to 3/4 inch of the dough clear on all sides. Pinch and twist the dough closed at the end. Place the bun pleat side down on the prepared baking sheet. Repeat the process with the remaining dough circles, spacing them 1-1/2 inches apart on the baking sheet. Loosely cover with plastic wrap or a kitchen towel. Set in a warm, draft-free place for 30 minutes to rise.
To bake the buns, about 10 minutes before the rising time is over, position a rack in the middle of the oven and preheat to 175°C (350°F).
Brush the top and side of each bun with the egg right before baking. Using the oven's convectional heat function, bake small buns for about 14 minutes and medium buns for about 18 minutes, or until rich golden brown; the cooked buns sound hollow when tapped at the bottom. Remove them from the oven, set on a rack, and let cool for 5 minutes. Brush the honey mixture on the buns for a sweet-glaze finish that will also soften the crust.