Rinse the beans with tap water and drain. Add the red beans into a big bowl along with 3 cups of water. Soak the beans overnight. (*Footnote 1)
Drain the beans and transfer them to a medium-sized pot. Add 8 cups of tap water. Heat over medium heat and stir occasionally to prevent the beans from sticking to the pan. When the water start to boil, turn to low heat. Simmer, covered, until the beans turn tender and start to fall apart, about 1 hour. Remove from heat.
(Optional) If you like a creamy soup, use an immersion blender to blend the beans until it reaches your desired consistency. Normally, the soup is quite coarse and still contains bits of red beans. Or you can blend the soup until smooth if you prefer.
Add the salt. Add the sugar, 1 tablespoon at a time, and stir to mix well. Taste the soup until it reaches the desired sweetness. I usually use 2 tablespoons for a mild sweet taste for a regular dessert or afternoon snack. But I would ramp up the sugar to 4 tablespoons if I’m planning serve the dish at a dinner party.
(Optional) Add the rice cakes if using, turn the stove back to medium-low heat. Heat the rice cakes until just tender, 5 to 8 minutes, depending on the type and shape of the rice cakes you use.
At this stage, you can add a small amount of water to adjust the texture of the soup, if needed.
Once done, serve the soup hot or cold as a dessert. You can store the leftover soup in an airtight container in the fridge for 3 to 4 days.