Rinse the glutinous rice under cold water until the water runs clear. Soak the rice in water for at least 4 hours or overnight for best results.
Drain the soaked rice and steam it in a bamboo or metal steamer lined with cheesecloth for about 25-30 minutes, or until tender.
While the rice is steaming, combine the coconut milk, sugar, and salt in a saucepan over medium heat. Stir until the sugar dissolves, but do not let it boil. Reserve about 1/2 cup of the coconut mixture for serving.
Once the rice is cooked, transfer it to a large bowl and pour the remaining coconut milk mixture over it. Stir gently to combine and let it sit for about 15-20 minutes to absorb the flavors.
To serve, place a portion of the sticky rice on a plate, top with sliced mango, and drizzle with the reserved coconut milk. Garnish with sesame seeds or mung beans if desired.