1. FILLING: Steam yam on high heat for 20 minutes. Mash with potato masher or fork and mix well with sugar and wheat starch.
2. Heat up a few tablespoons of oil in a non-stick pot and cook the yam until it becomes a firm ball. Leave to cool. Weigh 20g of the paste and roll into a ball. Cover with tea towel to avoid the paste from drying.
3. DOUGH A: Combine all-purpose flour, icing sugar and shortening in a bowl and mix until incorporated. Add in water and vinegar and knead until it form a smooth dough. Wrap the dough with a cling wrap and leave it to rest for 20 minutes.
4. DOUGH B: Knead all-purpose flour with shortening until it formed a smooth ball. Cover and let it rest.
5. Divide both dough into 4 portions equally and wrap dough B into dough A.
6. Using rolling pin, roll the dough into rectangle shape and roll dough from the shorter end to the other end like Swiss roll.
7. Again, flatten the roll dough by pressing it with the rolling pin then roll to form a flat prolong rectangle shape. Next, roll the shorter end to the other end to form a Swiss roll.
8. Divide each roll into 4 portions.
9. Roll each dough into a round shape and place 1 yam ball in the center. Enclose the end by slowly push pastry dough to the end of the meeting point and press to seal.
10. Prepare a pot of oil and deep fry the mooncake until golden brown.