Bear in mind that making the filling will require letting the seeds soaking overnight.
To start, cut the lotus seeds in halves and remove their hearts. They look like green stems and taste bitter. If you leave them, they will ruin the mooncakes' taste. In China, the lotus seeds' hearts are popular for making medicinal tea. You can save them for later and try brewing tea with them.
Place them in a bowl. Fill the container with water until it's at least 2 inches above the seeds. Then, cover the bowl with a kitchen towel and let the seeds soaking overnight.
Drain the water and transfer the seeds to a pot. Again, add water until you fill the pot 2 inches above the seeds.
Put the lotus seeds to a boil. Once it starts boiling, put it on low heat and let them cook without the lid for 30 minutes. You'll know they're ready when they're soft enough to squeeze them.
Drain the water and make the paste using a food processor or a blender. Save some of the water, in case you need to use it for making the paste.
To get a smooth paste, stop the food processor now and then to help it with a spoon. You can add some of the water you saved to make it easier. Keep processing until you get a hummus-like consistency.
Cook the paste on a non-stick pan over medium heat. After five minutes, the paste will get a bit dry. Then, it's time for adding icing sugar and oil slowly.
Now play your favorite playlist because you'll have to mix and cook non-stop for 30 minutes. When the paste feels more solid and keeps its shape, it's done. In my case, it took a bit less than 30 minutes. Then, let it rest at room temperature.