Combine the cornstarch, all-purpose flour, and half the granulated sugar in a small bowl, then briefly stir the yolks in a separate large bowl. The dry mixture should look evenly blended and the yolks should lose their separate outlines.

Warm the milk and remaining sugar in a small saucepan over medium heat, stirring occasionally, to about 120°F / 49°C without exceeding 150°F / 65°C. The sugar should dissolve completely while the milk remains below a simmer.

Whisk the dry mixture into the yolks while the milk heats. A uniform paste with no dry pockets is ready for tempering.

Whisk warm milk into the yolk paste in additions of about 1/4 cup until all the milk is incorporated. The thick paste should gradually loosen into a smooth, pourable mixture.

Cook the mixture in the saucepan over medium heat, scraping the bottom continuously, then maintain a bubbling boil for one full minute while stirring. Briefly pause to check for bubbles in the center before starting the timer, since thickening alone does not mean the custard has finished cooking.

Take the saucepan off the heat and work in the butter and vanilla. The butter should disappear into the custard, leaving a glossy mixture.

Set a large mesh strainer over a deep plate lined with plastic wrap and pass the custard through it. The strained filling should have an even texture without firm egg or flour lumps.

Lay plastic wrap directly against the custard and refrigerate for 2 hours, or use the freezer for a 30-minute quick chill. The surface covering prevents a skin, and the chilled filling should feel thick rather than flowing.

Choose a heatproof bowl that fits inside the steamer, set a strainer over it, and combine all three wrapper flours in another bowl. The flour mixture should have a consistent white appearance.

Sift the confectioners sugar into the combined flours. Break up any sugar lumps so they cannot leave dry specks in the batter.

Stir the milk, vegetable oil, and vanilla together in a small bowl. The oil may separate slightly, so stir again as needed while adding it.

Make a well in the dry mixture, stir in the liquid 2 to 3 tablespoons at a time, and strain the finished batter into the heatproof bowl. The batter should run easily through the strainer rather than fall in thick dollops.

Bring 1 inch (2.5 cm) of water to a boil in the steamer pot, then steam the bowl with the steamer covered over medium-high heat for 25 to 30 minutes. A spoon pressed into the center should meet stiff dough with no remaining milky liquid.

Let the bowl cool for a few minutes, then scrape and work the dough with a spoon for 5 minutes. The cooked mixture should gather into a cohesive ball.

Knead the dough by hand on a clean, un-floured counter for 5 minutes, then portion into 25-gram balls, cover, and chill until needed. Use food-safe gloves once the dough is comfortable to handle and aim for a satin-smooth, slightly oily surface.

Use a spatula to smooth the chilled custard, roll it into 25-gram balls with disposable gloves, and refrigerate the portions for another 30 minutes. The filling should retain a rounded shape instead of slumping.

Flatten each dough portion into a thin round about 4 inches (9 to 10 cm) across, using parchment sheets if it sticks. Keep the thickness even so the wrapper can surround the filling without weak patches.

Center a custard ball on each round, bring the dough around it, pinch away excess at the closure, and set it seam-side down under a loose cover. The filling should be fully enclosed with a smooth, unbroken top.

Fit the decorative plate into the mold and give the wrapped ball a light cornstarch coating. The surface should feel lightly dusted, with no thick white patches.

Position the smooth side against the pattern plate, press the mold firmly onto a flat surface, and hold for at least 30 seconds before releasing. Keep the plunger steady so the design forms without a doubled outline.