Soak the mung beans in water overnight, or at least for several hours.

Drain the beans and place them in a pot with enough water to cover them by about 2 inches.

Add the pandan leaves and bring to a boil.

Cook over medium-low heat until the beans are very soft and break apart easily.

Stir occasionally and add a little water if needed to prevent the beans from drying out too quickly.

Once soft, remove the pandan leaves and blend the beans into a very smooth paste.

Transfer the paste to a non-stick frying pan or wok and cook over low heat.

Add the sugar and salt, then stir continuously until the mixture starts to thicken.

Add the oil gradually, a little at a time, stirring well after each addition until fully absorbed.

Continue cooking until the paste becomes smooth, thick, and no longer sticks to the pan.

Let the paste cool completely, then chill it in the fridge before using it as mooncake filling.