Fry shallots in the cooking oil till aromatic. Add in the mashed yam, sugar and wheat starch and stir fried till the mixture is thick and mashy. Turn off the flame and puree the mixture in a food processor till the texture is smooth.
Divide the mixture into 30grams each and shape round. Set aside.
Combine the ingredients for the flour skin in a mixer and mix till all are well combined. Remove the dough and further knead it by hand for another 5 minutes by hand on a floured tabletop till it is soft and pliable. Roll the dough into a square.
Combine the ingredients for the oil skin in a mixer and mix till well combined. Remove the oil skin dough and further knead it by hand on a sparingly floured tabletop till it is not sticky anymore. Roll the oil skin into a square that is smaller than the flour skin and place it in the centre of the flour skin from Step 3.
Enclose both ends of the flour skin dough together and cover the oil skin entirely. Rest for 20 minutes.
Use a rolling pin to flatten all sides. Roll it into a swiss roll. Flatten the dough into a square, turn it 180 degrees and roll it into a swiss roll. Again, flatten the dough, turn it another 180 degrees and roll it into a swiss roll. Cut the swiss roll evenly into 25 pieces and wrap in the yam filling from Step 2. Seal the edge facing downwards.
Place the yam mooncake from the previous step on a baking tray lined with greaseproof paper. Bake in a preheated oven at 180 degrees C for 25 minutes till cooked (skin colour should be more on the white side to very slight brownish).