Rinse the sticky rice under cold water until the water runs clear. Soak the rice in water for at least 4 hours or preferably overnight. This will help soften the rice and ensure it cooks evenly.
After soaking, drain the rice thoroughly in a colander or sieve.
Prepare a steamer by filling a pot with water and placing a steaming basket or sieve lined with cheesecloth over it. The water should not touch the bottom of the steaming basket.
Transfer the drained sticky rice to the cheesecloth-lined steaming basket. Spread it out evenly.
Cover the steamer with a lid and steam the sticky rice over medium-high heat for about 25-30 minutes or until the rice is cooked and tender. Check it occasionally and add more water to the steamer if necessary.
While the rice is steaming, prepare the coconut sauce. In a separate saucepan, combine the coconut milk, sugar, and salt for the sauce. Stir over medium heat until the sugar dissolves, but do not let it boil. Once the sugar is dissolved, remove the saucepan from the heat and set the coconut sauce aside.
When the sticky rice is cooked, transfer it to a mixing bowl. While the rice is still hot, pour about 3/4 cup of the coconut sauce over the rice. Mix gently to coat the rice evenly with the coconut sauce. Let the rice sit for about 15 minutes to absorb the coconut flavor.
In a small saucepan, warm the remaining coconut sauce over low heat. This will be served as an additional sauce for the dish.
To serve, place a portion of the coconut-infused sticky rice on a plate or in a shallow bowl. Arrange sliced ripe mangoes on the side or on top of the rice.
If desired, sprinkle some sesame seeds or mung beans on top for added texture and presentation.
Serve the Mango Sticky Rice while the rice is still warm and the mangoes are ripe and juicy. Drizzle some warm coconut sauce over the top.