1. **Prepare the Faux Custard Yolk:** In a saucepan, whisk together custard powder, milk, sugar, cornstarch, and yellow food coloring (if using). Heat over medium-low heat, stirring constantly, until the mixture thickens into a firm custard. Remove from heat, stir in butter until melted and fully incorporated. Transfer to a bowl, cover with plastic wrap directly on the surface to prevent a skin from forming, and chill in the refrigerator for at least 1 hour, or until firm enough to handle. Once chilled, divide the custard into 12 equal portions and roll them into small balls. 2. **Prepare the Snowskin Dough:** Sift glutinous rice flour, rice flour, wheat starch, granulated sugar, and matcha powder into a heatproof bowl. Gradually whisk in the milk and condensed milk until a smooth batter forms. Stir in the vegetable oil. Cover the bowl tightly with plastic wrap or a plate. 3. **Steam the Dough:** Steam the covered bowl over high heat for 20-25 minutes, or until the dough is cooked through and translucent. Stir the dough halfway through the steaming process to ensure even cooking. Carefully remove the bowl from the steamer. 4. **Knead the Snowskin:** Let the dough cool slightly until it's comfortable to handle but still warm. Lightly grease your hands with a little oil or wear gloves. Knead the dough vigorously in the bowl or on a clean surface for 5-7 minutes until it becomes smooth, elastic, and non-sticky. 5. **Assemble the Mooncakes:** Dust your work surface, your hands, and your mooncake mold lightly with cooked glutinous rice flour. Divide the snowskin dough into 12 equal portions. Flatten one portion of the dough into a thin disc. Place a chilled custard yolk ball in the center. Carefully wrap the dough around the filling, pinching the edges to seal completely. Roll into a smooth ball. 6. **Mold the Mooncakes:** Place the sealed ball into your mooncake mold and press down firmly to create the desired shape. Gently release the mooncake from the mold. Repeat with the remaining dough and filling. 7. **Chill and Serve:** Arrange the finished snowskin mooncakes on a plate or in an airtight container. Refrigerate for at least 1 hour before serving to allow them to firm up. Enjoy your homemade mini matcha snowskin mooncakes!