Prep The Fruit: Peel the fresh rambutans and slice the juicy flesh away from the hard inner seeds. Discard the seeds and set the fruit pieces aside.
Whisk The Base: Pour the coconut water into a saucepan or pot. Add the agar agar powder and whisk it thoroughly until entirely combined and no lumps remain.
Infuse and Simmer: Add the peeled rambutan pieces into the pot with the liquid. Bring the mixture to a simmer over medium heat and let it simmer for 5 minutes. This is a crucial step because it activates the gelling properties of the agar agar while simultaneously drawing out the fragrant rambutan flavor into the liquid.
Separate The Fruit: Turn off the heat. Using a slotted spoon, remove the rambutan pieces from the liquid and add them to the bottom of a glass bowl or mold.
The Two Step Pour: To get your rambutan fruit pieces beautifully positioned on top, pour just a small amount of the hot liquid into the mold—just enough to cover the rambutan pieces. Place the mold into the freezer for 10 minutes to let this base layer set.
Unmold and Serve: Once set, gently run a toothpick or a thin knife around the outside edge of the mold to separate the jelly from the glass. Place a serving plate upside down over the top of the bowl, firmly hold both, and flip them over to release the pudding. Slice into clean pieces and serve cold!