Rinse the lotus seeds thoroughly and remove the green centers to prevent bitterness. Place the cleaned lotus seeds in a pot, add enough water to cover, and bring to a boil. Reduce to a simmer and cook until the lotus seeds are tender, then transfer them to a bowl. Be careful not to overcook so the seeds stay whole. In another pot, add the rock sugar and salt to the lotus seed cooking water, then heat over medium until the sugar dissolves. Add the lotus seeds back into the sweetened water and simmer on low heat until the seeds absorb the sweetness. Taste and adjust the sweetness as desired, then ladle into bowls.

To make the grass jelly:
- Mix the grass jelly powder and sugar together in a bowl, then add to a pot with the filtered water. Stir well to dissolve and let the mixture rest for 30 minutes.
- Place the pot on the stove and cook over medium heat, stirring constantly until the mixture boils and thickens. Lower the heat and cook for another 3–5 minutes, then pour the mixture into a mold.

Once the grass jelly has cooled, refrigerate until set, then cut into bite-sized pieces.

Spoon the black grass jelly and lotus seeds into bowls. Add more sugar to adjust sweetness if desired. Serve with ice for a chilled dessert!