Combine agar-agar powder, instant jelly, sugar, thick coconut milk, water, salt and pandan leaves in a saucepan and bring to a boil.
Turn off the stove after the mixture has boiled then add the milk and colouring.
Keep stirring the mixture until it feels just warm to the touch before poring into chilled mooncake moulds.
Preparing the moulds: Brush a little corn oil onto the wooden mooncake moulds. You may find two tiny holes at the sides of the mould (these are air vents) - stuff them with rolled bits of absorbent kitchen paper towel to prevent the warm jelly from seeping out. Keep the moulds well chilled in the freezer compartment for a couple of hours or preferably overnight. These moulds have to be really cold before the agar-agar mixture is poured in.
Assembling the mooncake: Pour the just warm agar-agar mixture into the prepared moulds to two-thirds full. Carefully immerse a piece of filling (either red dates or durian) into the warm mixture. Pour in more mixture to fill up the mould to the brim.
Leave the mould to set for five minutes then let it set completely in the freezer. Knock out the jelly mooncakes from the moulds. Keep the mooncakes chilled in the refrigerator before serving or packing them into boxes as gifts to relatives and friends.