For Red Bean Paste: Rinse soaked red beans. In a pot, combine red beans and 3 cups water. Bring to a boil, then reduce heat and simmer for 1-1.5 hours until very soft. Drain excess water, then mash beans until smooth. Stir in sugar and vegetable oil. Cook over medium-low heat, stirring constantly, until the paste thickens and pulls away from the sides of the pot. Let cool completely.

Once cooled, divide red bean paste into 15-20 gram balls, depending on your mooncake mold size.

For Mooncake Skin: In a bowl, whisk together glutinous rice flour, wheat starch, powdered sugar, and milk powder (if using). Gradually add cold water and melted shortening/oil, mixing until a soft, pliable dough forms. Knead lightly for 5 minutes until smooth.

Divide dough into balls, each weighing 25-30 grams, or slightly more than your filling balls. Flatten one dough ball into a disk. Place a red bean paste ball in the center. Carefully wrap the dough around the filling, sealing completely. Ensure there are no cracks.

Lightly dust a mooncake mold with glutinous rice flour. Place the filled dough ball into the mold. Press down firmly to create the mooncake shape. Gently release the mooncake from the mold.

Repeat with remaining dough and filling. Arrange mooncakes on a plate. Cover and chill in the refrigerator for at least 1-2 hours before serving for best texture.