Make creamy custard filling. In a small pot over slowest heat, dissolve Granulated Sugar (1/2 cup) with Milk (100 milliliter).
Sift all the Wheat Starch (1/2 cup), Cake Flour (2 tablespoon), and Custard Powder (1 1/2 tablespoon) quickly. Stir to combine well. Add Egg (2) and Unsalted Butter (2 tablespoon) in. Keep stirring forcefully in the whole process to combine everything and avoid caking.
Heat until the mixture becomes very thick and form a paste texture. Remove from fire and keep stirring for several minutes until it become fine and smooth.
Transfer out, cool down and cover with plastic wrapper. Refrigerate for several hours until hardened so you can shape the filling easily.
Combine Milk (185 gram), Granulated Sugar (40 gram), and Vegetable Oil (18 gram) well and then mix with Wheat Starch (20 gram), Rice Flour (35 gram), and Sweet Glutinous Rice Flour (45 gram).
Strain once and set aside for 30 minutes.
Cover with plastic wrappers and steam over high fire for around 30 minutes until it becomes slightly transparent.
Transfer out and stir with chopsticks forcefully for several minutes until fine and smooth. Transfer to a plate and cover with plastic wrapper. Knead with both hands for several minutes until the surface becomes oily. Refrigerate for at least 4 hours before assembly.
In a fry pan, stir fry Sweet Glutinous Rice Flour (1/3 cup) on medium low heat until the flour turns light yellow. Remove it from the heat. Set aside to cool down.
Measure each wrapper around 20 grams and filling 30 grams. Then shape both the wrapper and filling to balls.
Prepare the dusting flour, filling, wrapper, and mooncake stamp. Assemble the mooncake stamp well and then dust with flour. Shake off extra flour and set aside.
Wrap the filling with wrapper and seal completely. Then shape to a ball firstly and further to an oval. Place the oval to stamp, unsealed side first, flat with bottom with fingers; push the shaping tool to stamp the flowers.
Then demold carefully. If you find the mooncake is sticky on the stamp, use the other hand to help separate.