Prepare the Custard Filling: In a heatproof bowl, whisk egg yolks, granulated sugar, custard powder, milk, coconut milk, and vanilla extract until smooth. Place the bowl over a pot of simmering water (double boiler) and continuously stir until the mixture thickens into a rich, creamy custard. Remove from heat, stir in the melted butter until fully incorporated. Cover with plastic wrap, pressing it directly onto the surface to prevent a skin from forming, and chill in the refrigerator for at least 1-2 hours until firm.

Prepare the Snow Skin Dough: In a large bowl, whisk glutinous rice flour, rice flour, wheat starch, and powdered sugar. Gradually add milk and vegetable oil, stirring until a smooth, lump-free batter forms. Strain the batter through a fine-mesh sieve into a heatproof dish to ensure extra smoothness.

Steam the Dough: Cover the dish with plastic wrap and steam over high heat for about 20-25 minutes, or until the dough is cooked through and translucent. Carefully remove from the steamer and let it cool slightly. While still warm, knead the dough until smooth and elastic (about 5-10 minutes, use gloves if it's too hot). Let it cool completely.

Assemble the Mooncakes: Once both the custard filling and snow skin dough are chilled, divide the custard filling into 25-30g portions and roll them into balls. Divide the snow skin dough into slightly larger portions (around 30-35g).

Flatten a piece of snow skin dough into a thin circle, place a custard ball in the center. Carefully wrap the dough around the filling, sealing it completely. Gently roll it into a smooth ball.

Lightly dust your mooncake mold with toasted glutinous rice flour (excess can be brushed off). Press the mooncake ball into the mold, then invert and gently push it out to reveal the patterned mooncake. Repeat with the remaining dough and filling.

Chill and Serve: Arrange the finished mooncakes on a plate, cover loosely, and refrigerate for at least 2-4 hours before serving. They are best enjoyed chilled.