Prepare the Custard Filling: In a saucepan, whisk egg yolks, granulated sugar, and cornstarch. Gradually whisk in 120ml milk until smooth. Cook over medium-low heat, stirring constantly, until thickened. Remove from heat, stir in butter and vanilla until melted and smooth. Cover with plastic wrap directly on the surface to prevent a skin from forming. Chill until firm, at least 2 hours.

Prepare the Snow Skin Dough: In a large bowl, whisk 100g glutinous rice flour, rice flour, wheat starch, and powdered sugar. Gradually add 240ml milk and vegetable oil, whisking until smooth. Strain the mixture to remove any lumps.

Steam the Dough: Pour the strained batter into a shallow, heatproof dish. Steam over high heat for 20-25 minutes, or until the dough turns translucent and firm. Stir it halfway through to ensure even cooking. Let it cool slightly.

Knead and Cool: Once cool enough to handle, transfer the dough to a clean surface lightly dusted with cooked glutinous rice flour. Knead for about 5-10 minutes until smooth and elastic. Divide the dough into equal portions (e.g., 8 portions for medium mooncakes).

Assemble Mooncakes: Take one portion of snow skin dough and flatten it into a disc. Divide the chilled custard filling into equal balls and place one in the center of the dough. Carefully wrap the dough around the filling, sealing it completely.

Shape Mooncakes: Lightly dust a mooncake mold with cooked glutinous rice flour. Place the filled dough ball into the mold and press down firmly to create the desired pattern. Gently remove the mooncake from the mold.

Chill and Serve: Repeat with remaining dough and filling. Place mooncakes in an airtight container and refrigerate for at least 1-2 hours before serving. Enjoy chilled!