Che Dau Xanh (Vietnamese Dessert Soup With Mung Beans)

1 cup dried mung beans, soaked for at least 4 hours
5 cups water
1/2 cup sugar
1 (14-ounce) can coconut milk

Drain and rinse the soaked mung beans.
Place beans and 5 cups water in a large saucepan. Bring to a boil, add sugar, and stir. Reduce heat to low and simmer until beans are tender, about 45 minutes to an hour.
Turn off heat and stir in coconut milk.
Let cool, then refrigerate for at least an hour.