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To make the custard: In a medium saucepan over medium-low heat, bring the milk, cream, and ginger to a bare simmer — you should see the liquid steaming with some tiny bubbles forming around the edges.
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Meanwhile, in a large bowl, whisk the sugar, cornstarch, and salt until well-combined and lump-free. Add the egg yolks and whisk vigorously until the mixture is smooth, airy, and pale, about 1 minute.
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When the milk mixture is hot, slowly dribble it into the egg yolk mixture, whisking constantly. This process of slowly adding hot liquid to the eggs yolks is called tempering and ensures that the eggs don’t scramble from sudden heat. After you’ve slowly added about 1/3 cup of the hot milk, you can add the rest faster. Once it is all incorporated, transfer the liquid back into the pot, set over medium-low heat, and cook, whisking constantly, until the mixture starts to simmer and thicken to a pudding-like consistency, about 3 to 4 minutes.
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Pour the custard through a fine-mesh sieve, pressing it through with a spatula or spoon, and into a container. Stir in the vanilla. Place a piece of plastic wrap against the surface of the custard to prevent a skin from forming, then refrigerate for at least 2 hours. While the custard chills, make the dough.
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To make the dough: In a medium bowl, combine all the dough ingredients and knead — by hand or with a mixer — to make a soft, elastic dough, about 8 to 10 minutes in a stand mixer. Place the dough in a lightly greased bowl, cover, and let it rise for 60 to 90 minutes, until it's nearly doubled in bulk.
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To shape the buns: Cut ten 4" squares of parchment (alternatively, use steamer liners). Turn out the dough onto a lightly floured surface and divide into 10 equal pieces (around 68g per piece).
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Shape each piece into a smooth, tight ball, placing the shaped buns on their own parchment square.
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Place the dough balls, on their parchment squares, into the steamer basket, leaving plenty of room in between for the buns to expand. (You’ll be able to fit 3 buns into each layer of a 10" steamer basket.). Transfer any buns that don’t fit onto a plate or baking sheet. Cover all of the buns and let them rise for 30 to 40 minutes, until puffy and slightly larger.
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To steam the buns: Bring a shallow pot or frying pan of water to a boil. Place steamer basket over the top and steam for 10 minutes. Remove the steamer basket from the water and let the buns rest, still covered, for 10 minutes. Finish steaming any remaining dough balls.
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To fill the buns: Once the buns are cool, cut them lengthwise through the center (the opposite of how you’d cut a burger bun), being sure not to slice all of the way through. Gently pry apart the two sides to expand the opening, then use a small spoon or a pastry bag to fill the bun with about 2 to 3 tablespoons (30g to 40g) of chilled custard. Use an offset spatula to smooth the custard flush against the bun’s surface. Press julienned or diced ginger into the custard. Serve right away.
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Storage instructions: These buns are best served immediately. If you’d like to fill only a few buns at a time, save the steamed, uncut buns and the extra custard in separate airtight containers in the refrigerator for up to 3 days. To reheat the buns, wrap in a damp paper towel and microwave in 15-second increments. Let cool slightly, then split open and fill with the chilled custard.
