For the Custard Filling
In a small pot, combine custard powder, wheat starch, cake flour and granulated sugar together. Heat over low heat and then add milk. Stir continuously until all ingredients mixed well.
In a mixing bowl, crack 1 egg and add in 1 egg yolk and beat using a fork. Stir in beaten egg mixture slowly into the pot, constantly stirring while throughout the time. Next, add in butter and heat until well mixed. Remove the mixture from the heat and fold the custard over and over again using your hands, until it forms a smooth ball.
Place the custard filling in fridge for at least 1 hour or until hardened.
For the Bun Dough
Add sugar and yeast to the warm water, leave it for around 10 minutes until the yeast is well activated. Mix in all other ingredients to the warm yeast water. Combine everything together and knead until the dough becomes smooth.
In a large bowl, brush on some oil and transfer the dough in for proofing. Let it sit for 2-3 hours until the dough raises and doubles in size.
For the Custard Buns
When the dough is doubled in size, transfer to a slightly floured board. Using your hands, knead the dough until the surface becomes very smooth. Roll the dough until it forms a long log, then cut into 12 equal portions.
Divide the hardened custard filling into 12 equal portions and shape each portion in to a small ball.
Return back to the dough, take one single portion and press down slightly. Then using a rolling pin, roll the dough into a round wrapper, 10cm in diameter, edges thinner than the centre.
Place one portion of the custard filling in the centre of the round dough wrapper and then seal completely. Slightly wet the tip of a toothpick to attach the black sesame seeds as the eyes of the bunnies. Repeat the process until all buns are finished.
Set up the wok or steamer on high heat, place buns in to steam for approximately 15-20 minutes. Turn heat off and let it sit in the steamer for a further 5 minutes. Remove from heat and slit the surface of the bun with a small knife to form the ears. Serve immediately.