1. Sift cake flour, hk flour, and salt into a mixing bowl.
2. Combine castor sugar, yeast and water with the flours, and blend to mix well- Then knead until soft dough is formed.
3. Add in shortening, and continue to knead until dough is smooth and elastic.
4. Prove dough for 60-90mins until dough has risen to its limit.
5. Meanwhile, prepare custard filling- Combine custard powder, wheat starch, and castor sugar in a nonstock pan.
6. Add in milk, and stir mixture over low heat until mixed- then add butter.
7. When butter is dissolved, turn off heat, and keep stirring while gradually adding the egg.
8. Divide dough into 12 portions, shaped into balls, cover with damp cloth, and set aside.
9. Using a rolling pin, roll the a ball where the edges are thinner than the center.
10. Add custard filling in the middle, and wrap to form a bun.
11. Place on parchment paper, and repeat for the other buns.
12. Prove for 15-20mins.
13. Boil water in steamer, and steam custard buns over rapid boiling water for 10-15mins- Enjoy!
14. NOTE: Hongkong flour is to make the buns white , like the restaurants' buns.
15. However, if you don't have any.
16. you can use all purpose flour, but it wont be as white.