Soak the peanuts overnight in water, ensuring that all the peanuts are fully submerged.
The next day, rinse the peanuts thoroughly and transfer them to a pressure cooker with 8 cups of water. Depending on your pressure cooker, you may want to adjust the amount of water accordingly.
Use high heat to pressure cook the peanut soup for 15 minutes, then reduce the heat to medium and cook for an additional 30-40 minutes. Turn off the heat and allow it to cool until you can safely remove the lid of the pressure cooker.
Reheat the sweet peanut soup over medium to low heat and add rock sugar to taste. Boil until the peanuts are thoroughly soft. Add the coconut milk towards the end, boil for 1 minute, then turn off the heat and serve immediately.
Notes
The water should reduce to about 6-7 cups after they are done. The soup should be creamy in color, not too watery, but creamy and rich.