Place walnuts and black sesame seeds on a dry pan and toast them over medium-low heat until fragrant, about 5 minutes or so.
Place the walnuts, black sesame seeds, and half of the water in a blender and blend until they are smooth
Cooking:
Pour into a pot and the rest of the water, rock sugar, knotted pandan leaves, and pinch of salt. Turn on the stove and set the heat to medium
Keep stirring. It seems like nothing is going to happen for a while, but just keep stirring. Stirring also helps to prevent scorching at the bottom of the pot. The mixture will start to bubble and gradually thicken, it takes about 8-10 minutes. Keep in mind that it thickens further and the color darkens as it cools down
It should coat the spatula thickly and when you draw the line on the spatula, it stays
Remove the knotted pandan leaves and scrape the walnut and black sesame mixture that clings to the leaves into the pot (I don't like to waste food LOL)
Serve warm or at room temperature
How to store:
Let it cool down completely. It will form a layer of skin on top as it cools down. Not a problem, you can just stir it in. Store in an air-tight container in the fridge for 3 days. Simply reheat in the microwave or on the stove until heated through. They are super thick when they are cold from the fridge. The heat will "melt" the mixture. You can always add a bit more water if you find it a bit too thick after reheating