For water dough: Sift together plain flour and icing sugar, add in salt, mix well. Add in lard, rub into flour mixture until well mixed. Add in cold water, mix into a dough. Knead the dough for 5 minutes until smooth (coat with flour if the dough is sticky). Wrap with cling wrap, set aside to rest for 30 minutes. (Reminder: The water absorption capacity for different brand of flour is different, if your dough is drier, add in more cold water until it reaches the consistency as shown in the video. You may replace lard with pure ghee/clarified butter or shortening.)
For oil dough: Sift plain flour, add in lard, mix until a dough is formed. Cover the dough, keep refrigerated while waiting for the water dough to rest. (Reminder: If your oil dough is drier, add in more lard. You may replace lard with pure ghee/clarified butter or shortening.)
Divide water dough and oil dough into 6 portions. Flatten water dough, top with oil dough, wrap oil dough fully with water dough, shape into a ball.
Lightly coat dough with flour, roll out the dough in between cling wrap into oblong shape, then roll up. Roll out the rolled dough again, then roll it up again.
Cover the dough with cling wrap, set aside to rest for 20 minutes.
Steam salted egg yolks for 5 minutes. Set aside to cool.
Divide black sesame paste into 6 portions (40 g each).
Shape black sesame paste into a bowl, fill with salted egg yolk, wrap the egg yolk fully with the paste. Press until firm, then shape gently into a ball.
Flatten rested dough into slightly round shape, put black sesame paste at the center, wrap fully with dough.
Arrange filled dough on a baking tray lined with non-stick baking paper, bake in the preheated oven at 200°C/390°F for 20 minutes.
For egg wash: Mix egg yolk and water, sieve through.
After baking for 20 minutes, brush the surface with egg wash and top with some black sesame seeds.
Turn the tray, bake again at 200°C/390°F for 8 minutes, until golden brown.
Set aside the black sesame mooncakes to cool completely before serving.