a 4-inch ring cutter; a piping bag fitted with a medium plain tip
For the roast pork: Combine the honey, hoisin, soy sauce, rice wine, sesame oil, ginger, five-spice powder and garlic in a nonreactive baking dish large enough to hold the pork snugly. Whisk until smooth and season with pepper. Cut the pork against the grain into 1-inch-thick slices. Add to the marinade and toss to coat. Cover and refrigerate at least 4 hours and up to 8 hours.
Preheat the oven to 350 degrees F. Line a rimmed baking sheet with foil and place a rack on top.
Drain the meat from the marinade (reserving the marinade) and place on the rack. Roast until the top is lightly caramelized, about 25 minutes. Flip the pieces and roast until the second side is lightly caramelized, about 25 minutes more.
Meanwhile, pour a few tablespoons of the marinade into a small bowl. Add the cornstarch and whisk until smooth with no lumps. Add 2 cups of the marinade to a small saucepan (discard the rest) and heat over medium. Add the cornstarch mixture to the saucepan. Bring to a boil, then reduce to a simmer and cook until thickened, about 1 minute. Remove from the heat and set aside to cool to room temperature, about 15 minutes.
Let the pork cool, then chop into small pieces for the filling and put in a medium bowl. Add the thickened, cooled marinade and stir to coat.
For the dough: Combine the yeast, 1/2 cup warm water (90 to 110 degrees F) and a pinch of sugar in a large spouted measuring cup. Let sit until foamy, about 3 minutes.
Meanwhile, combine the all-purpose flour, cake flour, granulated sugar and salt in the bowl of a mixer fitted with the paddle attachment. Whisk the milk, vegetable oil and egg into the yeast mixture. Add to the flour mixture and mix with the paddle to make a cohesive dough. Switch to the dough hook and knead, adding a little more flour or water if needed, to make a dough that cleans the sides of the bowl. Knead until the dough is very smooth and springy, 4 to 5 minutes. Knead the dough on a floured counter to bring the dough together. Oil a large bowl and turn the dough to coat in the oil. Cover and let rise until doubled in size, about 1 hour.
Line 2 baking sheets with parchment. Punch the dough down and divide into 2 pieces. Roll each piece to about a scant 1/4 inch thickness and use a 4-inch ring cutter to cut as many circles as possible. Reroll the dough to cut out 9 to 12 circles total. Fill the center of each with about 1 tablespoon of the filling. Pinch the dough to enclose the filling and make a ball. Put the balls pinched-side down on the baking sheets. Cover loosely with plastic wrap and let rise at room temperature for 30 minutes.
Preheat the oven to 350 degrees F.
For the topping: Combine the flour, confectioners' sugar, baking powder and salt in the work bowl of a food processor. Pulse to combine. Add the butter and process until crumbly. Add the egg and egg white and process to make a very smooth paste. Transfer to a piping bag fitted with a medium plain tip.
Once the buns have risen, pipe a spiral of topping on each bun. Bake until the buns and topping are golden, 20 to 22 minutes. Serve warm.