(Optional step) If you cannot find toasted black sesame seeds, place them in a frying pan and heat on medium heat until the seeds start to jump, like popcorn.
Grind toasted black sesame seeds into powder.
To toast glutinous rice flour, place them in a frying pan and heat on medium heat until slightly browned, stir every so often.
Boil 6 cups of water in a sauce pot, then add ground black sesame seeds, toasted glutinous rice flour, and rock sugar. Turn the fire to low and stir well until all the sugar has melted, then turn off heat. If there are still clumps after all the sugar has melted, you can use immersion blender (or regular blender) to puree the soup into a super smooth texture. Serve hot.