Toast black sesame seeds on a dry pan over low heat until aromatic, about 10 minutes. Remove from the heat and let them cool down completely. You can also purchase the toasted black sesame seeds and skip this step
Put the toasted black sesame seeds in a grinder or blender and pulse a few times to turn into a powder
Put the black sesame seeds powder and the rest of the ingredients into a pot. Stir to combine everything. Alternatively, you can also put the sesame seeds along with everything else, except for rock sugar in the blender and blend until smooth. There will still be specks of sesame seeds
Pour into a pot on the stove and cook over medium-low heat and keep stirring
It seems like nothing is going to happen for a while, but just keep stirring, the glutinous rice flour will thicken the mixture gradually. Stirring also helps to prevent scorching at the bottom of the pot. It will thicken. Remove from the heat and serve warm
Video
Notes
300 ml of liquid gives a very thick consistency, more like a paste, which I like. You can play with the amount from 300 ml to around 400 ml. 500 ml of liquid gives a very soup consistency. Please adjust to your preference on how thick you want it.
If you don't have rock sugar, you can use brown or white sugar. Rock sugar is less sweet compared to white sugar and you need to adjust the amount according to your taste.