Step 1 Soak the rice in water for at least 1 hour, preferably overnight. (The grains of rice must absorb moisture before cooking.)
Step 2 Drain the rice. Line a bamboo steamer basket or the perforated bowl of a double boiler with a double layer of cheesecloth. Fill the base of the double boiler, or a wok if using the bamboo steamer, with water, making sure the top of the water does not touch the base of the steamer.
Step 3Bring the water to a boil. Reduce the heat to a gentle simmer and place the drained rice in the steamer. Cover and cook for about 15 minutes until done (it should be softer than al dente).
Step 4While the rice cooks, heat the coconut milk in a small saucepan over medium heat just until it begins to simmer, then turn off the heat. (Boiled coconut milk will curdle and separate.) Add the sugar and salt to the coconut milk, and stir until it's dissolved.
Step 5When the rice has cooked, remove it to a medium bowl, and gently stir in half of the seasoned coconut milk using a spatula or wooden spoon. Cover and set aside for 5 minutes to allow the rice to absorb the coconut milk.
Step 6Peel the mangoes, and slice each cheek off the pit. Slice each cheek lengthwise in half and then crosswise into three-fourths-inch-thick slices. Into each of four small bowls, spoon one-half cup of the sticky rice, and gently drizzle the remaining coconut milk over each serving, about a tablespoon each. Arrange each sliced cheek of mango over the rice and sprinkle the top with black sesame seeds. Serve immediately.