In a mixing bowl, whisk together the egg yolks, sugar, and whole milk until smooth. Heat the mixture gently over low heat, stirring constantly, until it thickens into a custard. Set aside to cool completely.

Once cool, stir in the cake flour, powdered milk, and unsalted butter (or vegetable oil) into the custard mixture until well combined and smooth. This will be your base filling.

In a separate bowl, combine the snow skin premix with the boiling water. Stir well until a dough forms. Knead briefly until smooth and pliable. Let it cool completely.

Divide the cooled snow skin dough into three equal portions. Knead 1 teaspoon of beet powder into one portion, 1 teaspoon of matcha powder into another, and 1 teaspoon of blueberry powder into the third portion until the colors are vibrant and evenly distributed.

If using, mix durian powder or your chosen flavor powder into a portion of the cooled custard filling.

For each mooncake, take small, equal-sized portions (e.g., 15-20g) of each colored snow skin dough. Gently combine them to create a marbled or layered effect, but do not fully mix the colors.

Flatten the combined colored dough into a thin disc. Place a small amount of the prepared custard filling in the center.

Carefully wrap the dough around the filling, pinching the edges to seal it completely. Roll into a smooth ball.

Dust your mooncake mold and the mooncake ball lightly with cooked glutinous rice flour to prevent sticking. Press the mooncake into the mold firmly to get the desired shape and pattern.

Gently release the mooncake from the mold. Repeat with the remaining dough and filling.

Arrange the finished mooncakes on a plate and refrigerate for at least 2-3 hours before serving to allow them to firm up.