MAKE THE TANGZHONG: In a small saucepan, whisk the flour and milk so that there are no lumps. Cook over medium heat, whisking constantly, until the mixture becomes elastic, like the consistency of thick glue, about 1 minute. Transfer the mixture to the bowl of a stand mixer and let it cool completely, about 10 minutes.

MAKE THE DOUGH: Butter a medium bowl. To the mixer bowl holding the cooled tangzhong, add the milk, flour, salt, egg, and yeast. With the dough hook attachment, mix on low until all the ingredients are combined, pausing to scrape the sides and bottom of the bowl with a rubber spatula. Increase the speed to medium and mix for 4 minutes more. Decrease the speed to low and sprinkle in the sugar, 1 tablespoon at a time. Gradually add the butter, one or two pieces at a time, waiting between each addition for them to be incorporated into the dough, then mix for 4 minutes more. The butter should be completely absorbed into a glossy and stretchy dough that is beginning to come off the sides of the bowl. Transfer the dough to the prepared bowl. Cover it with a kitchen towel or plastic wrap and chill it overnight (up to 12 hours) in the refrigerator or let it rest and rise at room temperature for 2 hours, until you are ready to shape it.

MAKE THE PASTRY CREAM: Add the milk to a small saucepan. Scrape the seeds from the vanilla bean into the milk, then add the scraped pod. Bring the mixture to a simmer over low heat, turning the heat off as soon as you see bubbles forming at the edge of the pan. Fish out and discard the vanilla pod. In a small bowl, whisk the egg yolks, granulated sugar, and cornstarch to a smooth and uniform paste. Add about ½ (120 g) cup of the scalded milk to the egg mixture and whisk together, then add this back to the pan with the rest of the milk. Bring to a gentle simmer over medium-low heat and cook, whisking continuously, for 2 to 3 minutes, until a thick pasty cream forms. Transfer the custard to a small bowl. Place a piece of plastic wrap directly onto the surface of the pastry cream and chill it for at least 30 minutes in the refrigerator.

MAKE THE BUNS: Line two 13 by 18-inch (33 by 46 cm) baking trays with parchment paper. Lightly dust a work surface with flour. Roll the dough into a 13 by 17-inch (33 by 43 cm) rectangle, gently pinching and pulling at the corners to square them off as much as possible. Using a ruler and a chef’s knife or pizza cutter, cut the dough into a 3 by 4-inch (7.5 by 10 cm) grid (for twelve rectangles total). In the center of each rectangle, place 1 heaping tablespoon of the pastry cream. Gather the corners of the dough together, pinching them so that you seal the custard inside the dough, almost as if you were making a dumpling. You can pick it up and cradle it in the palm of your hand as you seal it. Place the bun seam side down on one of the prepared baking trays. Repeat with the remaining rectangles of dough. You should have six buns per tray, spaced 2 to 3 inches (5 to 7.5 cm) apart.

PROOF THE BUNS: Cover the buns with a kitchen towel and set them in a warm and draft-free area until they are noticeably puffier, slightly dry looking, and the impression of a fingerprint is slow to fill in, 1 to 2 hours, depending on the temperature of your kitchen. When the buns are sufficiently proofed, preheat the oven to 350°F (175°C/ 160°C fan). Bake for about 20 minutes, rotating the baking trays halfway through from top to bottom and front to back, until pale brown. Transfer the buns to a rack to cool.

WHILE THE BUNS ARE COOLING, MAKE THE GLAZE: In a small saucepan over medium heat, bring the cream to a gentle simmer, turning the heat off as soon as you see bubbles forming at the edge of the pan. Whisk the chocolate into the cream, continuing to stir as it melts and forms a silky glaze. Dip the top of each cooled bun into the glaze and allow them to set before serving.