First, prepare the custard filling: In a heatproof bowl, whisk together egg yolks, 4 tablespoons sugar, and whole milk until smooth. Add cake flour and powdered milk, whisking until no lumps remain. Stir in the melted butter (or oil).

Place the bowl over a pot of simmering water (double boiler method) and continuously whisk the mixture until it thickens into a custard, about 8-10 minutes. Remove from heat and let it cool completely. If using, mix in durian powder or any other flavor powder into the cooled custard. Roll the cooled custard into small balls, about 15g each, and set aside.

Next, prepare the snow skin dough: In a large bowl, combine the snow skin premix with 1 cup boiling water. Stir well until a dough forms. Once cool enough to handle, knead the dough gently until smooth. Divide the dough into three equal portions.

In separate bowls, mix one portion of dough with beet powder, another with matcha powder, and the third with blueberry powder. Knead each portion until the color is evenly distributed. You should now have three distinct colors of dough.

Lightly dust your work surface and mooncake mold with cooked glutinous rice flour. Take a small piece (about 15-20g, or 1 tablespoon) of one colored dough. Flatten it into a round disc in your palm.

Place one custard filling ball in the center of the dough disc. Carefully wrap the dough around the filling, sealing it completely. Ensure there are no cracks and the filling is fully enclosed.

Gently place the filled dough ball into your mooncake mold. Press down firmly to imprint the design. Carefully unmold the mooncake onto a plate.

Repeat the process with the remaining dough and filling, alternating colors or creating mixed-color effects as desired. Chill the finished mooncakes in the refrigerator for at least 2 hours before serving for the best texture and flavor.