Rinse rice with tap water until the water runs clear. Usually 2 times, is good enough. Then, soak it in a bowl of water for at least 4 hours or overnight.
After soaking, drain the rice and place it in a cheesecloth-lined bamboo steamer (or any steamer you have on hand).
Steam over boiling water for 20 minutes, or until the grains are tender and translucent.
While the rice is steaming, combine coconut milk, sugar, and salt in a saucepan. Heat gently over medium heat, stirring until the sugar dissolves and the sauce thickens a bit. Reserve about ¼ cup of this mixture for drizzling later.
Once the rice is cooked, transfer it to a mixing bowl. Pour the remaining warm coconut sauce over the rice, stirring gently to coat. Let the rice absorb the sauce for 20–30 minutes. At this stage, you can cover and leave it at room temperature. It may look overly wet at first, but it will thicken beautifully as it sits.
Peel the mangoes and slice the flesh away from the pit. Cut into thin slices or cubes, depending on your presentation preference.
Scoop a generous portion of sticky rice onto a plate. Arrange mango slices on the side.
Drizzle with the reserved coconut sauce and, if desired, sprinkle with toasted sesame seeds or crunchy fried mung beans for texture. Serve warm or at room temperature.