Combine yam and pandan leaves in a steamer and steam over medium heat for 20 to 25 minutes, until yam is soft and a chopstick can poke through the yam easily. Discard the pandan leaves.
500 grams peeled yam, 2 blades pandan leaves
Transfer yam into a mixing bowl and add in sugar. Mash yam while it's still hot. Stir in half of the shallot oil. Blend the yam until very smooth (I used an immersion blender), adding as much Part 1 water as needed if the paste is very thick and stiff.
Cook the paste over medium heat, stirring constantly, to form a firm paste. This will take up to 15 to 30 minutes (see Note 1).
Line a baking sheet with parchment paper. Transfer the cooked yam paste onto the lined baking sheet. Cover the surface of the paste with clingwrap to prevent a skin from forming. Let cool completely.
The paste can be made up to 2 days in advance, keep paste frozen otherwise. Thaw the frozen paste the night before in the refrigerator before using.
For the mooncakes, divide the paste into 35-gram portions. Roll each portion into a ball. Cover and keep chilled until needed.
NOTES
To make shallot oil, thinly slice peeled shallots. Place shallots and neutral-flavoured oil (1:1.2 ratio by weight, i.e. 100 grams shallots to 120 grams oil) in a pot. Cook over medium-low heat, stirring occasionally. When the oil starts to foam, start stirring frequently until shallots are lightly golden. Strain before using.
I refer to this YouTube video for the consistency of the paste.