Put the 3 cups coconut milk and the water in a large saucepan and bring to a boil. Stir in the tapioca and bring to a boil, then reduce the heat to low and simmer, stirring, until the tapioca Becomes transulent and soft, 10 to 15 minutes for tiny pearls, longer for larger tapioca pearls. Meanwhile, slice the bananas lenghtwise in half, then cut crosswise in half; if using regular bananas, cut each half into thirds or quarters. Gently stirring, add the bananas, sugar and salt to the tapioca and cook for 10 minutes more. Remove from the heat. While the bananas are cooking, put sesame seeds in a small heavy skillet and dry-roast over medium-high heat, shaking constantly, until light brown and crisp, about 2 minutes. Remove from the heat. If using sesame seeds, pound briefly with a mortar and pestle or pulse in a mini-chop 2 or 3 times to release their flavor. Ladle the warm bananas and tapioca into shallow bowls, sprinkle with the toasted mung beans or sesame seeds and serve with extra coconut milk on the side.