Prepare the ingredients:
- Soak dried lotus seeds in water for 30 minutes, then simmer over low heat until tender but not mushy.
- Once the lotus seeds have cooled slightly, use a toothpick to remove any remaining lotus seed cores to prevent bitterness.
- Quickly rinse the dried longan with water (do not soak, as dried longan softens quickly when cooked).
- Using rock sugar gives the dessert a light, clean sweetness.

Make the white grass jelly:
- Prepare the white grass jelly according to the instructions on the package (for a firmer, chewier texture, use less water).
- Once cooked, pour the jelly into a container and let it cool, then refrigerate until set. When ready to serve, cut into cubes or strips.

Cook the sweet soup:
Pour 6 1/3 cups filtered water (about 1.5 liters) into a pot with the rock sugar and bring to a boil. Once the sugar dissolves, add the lotus seeds and dried longan. Simmer over medium-low heat until both are soft and plump. Adjust sweetness to taste, then turn off the heat and stir in the vanilla extract.

To serve:
- Transfer the sweet soup to a serving bowl. When serving, ladle into individual bowls. Add white grass jelly and ice for a cold dessert, or enjoy warm as you prefer.
- The lotus seeds are tender, the longan is chewy, and the white grass jelly is bouncy and lightly sweet—a fragrant, cooling dessert perfect for refreshing the body.

Notes:
- When transferring the sweet soup to a serving bowl, only include the lotus seeds and longan.
- Add the white grass jelly only when serving, according to personal preference.