Chè đậu đen (Vietnamese Black Bean Sweet Soup)

Rinse the dried black beans under cold running water. Place them in a large bowl and add the soaking water. Let soak for 8 hours or overnight. The beans will plump and their skins will wrinkle slightly. Drain and discard the soaking water.

Transfer the drained beans to a large pot. Add the cooking water and the optional salt. Bring to a boil over high heat, then reduce the heat to low, cover, and simmer gently for 40 to 50 minutes, until the beans are very tender and splitting. Stir occasionally and skim off any foam that rises to the surface.

When the beans are tender, add the knotted pandan leaves and the sugar. Stir gently until the sugar dissolves completely. Continue to simmer, uncovered, for another 10 to 15 minutes so the flavors meld and the liquid reduces slightly. Taste and adjust sweetness to your liking. Remove the pandan leaves.

If you prefer a slightly thicker broth, mix the mung bean starch or tapioca starch with 2 tablespoons of cold water to make a slurry. Stir the slurry into the simmering soup and cook for 2 to 3 minutes, until the broth is glossy and lightly thickened. Remove from heat. The chè will thicken further as it cools.

Ladle the chè into small bowls. Serve warm, or let cool to room temperature and serve over ice if you like. The beans should be soft but intact, suspended in a fragrant, gently sweet syrup. Leftovers keep in the refrigerator for up to 3 days; reheat gently on the stovetop or in the microwave.