Heat a wok over high heat until hot but not smoking.

Add sesame seeds and stir for 45 seconds-1 minute or until they stop crackling.

Remove from heat and stir in the wok for 15 seconds longer before transferring to a bowl to cool.

Grind the cooled sesame seeds and 1/4 cup of sugar until fine.

Add 1/4 cup of cold water and process to a smooth paste.

Mix the rice flour with a little of the remaining water to form a smooth paste.

Whisk in all of the remaining water.

Bring the water mixture to a boil over medium heat,stirring so no lumps form.

Crush the rock sugar and add to the pot along with the sesame paste.

Return to a boil and cook,stirring as needed,until the sugar dissolves and the soup thickens to the consistency of a thin porridge.

Serve 1 cup per person.