1. To make pandan juice, place all the leaves and water in a blender. Blitz for about 1 min, then strain the pandan pulp through a muslin cloth or fine strainer. If using a strainer, use the back of a spoon to press down on the pandan pulp to extract every last drop of juice. Discard pulp and use the juice accordingly. You can make this up to 3 days ahead and store in the fridge, covered.
2. Before you start making the jelly, have a rectangular tray (about 15cm x 24cm x 6cm) or a 20cm round tray ready by the stove. Also make sure you have space in the fridge for the bowl to sit on a sturdy flat surface.
3. To make the coconut layer, place all its ingredients, except the pandan essence and green colouring, into a saucepan over medium-low heat. Bring to a gentle boil, stirring constantly with a whisk. Keep stirring until the agar agar powder is completely dissolved, about 3 minutes. Remove from the heat and add the pandan essence and colouring, if using.
4. Stir to mix and gently pour into the tray. Remove any bubbles with a teaspoon, or pop them with a skewer. Place the tray in the fridge while you work on the coconut water layer.
5. To make the coconut water layer, place all the ingredients in a clean saucepan and bring to a gentle boil, stirring often, over medium heat. Once it comes to the boil, lower the heat and stir till the agar agar powder is completely dissolved, about 3 minutes.
6. Take the tray out of the fridge. The coconut layer should have set softly (the sides will set quicker than the middle). To check, use your finger to gently prod the centre of the tray. If it feels slightly set, then it is safe to start pouring the coconut water layer on top.
7. Don’t worry if some of the kaya layer starts to melt and mingle with the coconut water layer. Because the kaya layer is heavier, it will eventually settle to the bottom as it sets.
8. Remove any bubbles with a teaspoon, or pop them with a skewer. Let the jelly set in the fridge for at least 3 hours before serving, cold.