Rinse rice 3 times with water. Place the rice in a medium bow and soak in water for 1 hour. Drain.
In a medium pot, add 4½ cups water. Bring to a boil over high heat. Once boiling, add rice and cook for 15 minutes, stirring occasionally. Reduce heat to medium-low, cook for 30 minutes, stirring occasionally.
Add 80 grams rock sugar. When it melts, taste and adjust with more sugar if needed. Cook for 10 more minutes. Stir in the coconut milk.
For hot soup, transfer to serving bowls and serve immediately. Garnish with a little coconut milk if desired. For cold soup, set the pot aside for 30 minutes to cool. Transfer to a container and chill in the fridge for at least 2 hours. Garnish with a little coconut milk if desired.