Soak the rice: Rinse the sticky rice under cold water 3 to 4 times until the water runs mostly clear. Then, place it in a bowl, cover with water, and soak for at least 6 hours or overnight.
Prepare the steamer: Drain the soaked rice thoroughly. Line your bamboo steamer or metal basket with cheesecloth, then spread the rice evenly across it, making sure not to pile it too thick in the center.
Steam the rice: Bring water to a boil in a large pot, then place the steamer basket on top, ensuring the water doesn't touch the rice directly. Cover and steam for 20 to 25 minutes, flipping the rice halfway through, until the grains turn soft and translucent.
Make the coconut sauce: While the rice steams, combine 1 cup of coconut milk, 1/4 cup of sugar, and 1/2 teaspoon of salt in a small saucepan. Warm over low heat, stirring frequently, until the sugar dissolves completely. Do not let it boil.
Combine rice and sauce: Transfer the hot steamed rice into a large bowl. Pour two-thirds of the warm coconut sauce over it, then stir gently to combine. Cover the bowl and let it rest for 15 to 20 minutes so the rice absorbs the liquid.
Prepare the topping sauce: In the same saucepan, combine the remaining coconut milk, sugar, and salt. Heat gently until slightly thickened, then set aside to drizzle over individual servings.
Serve: Scoop the coconut sticky rice into serving bowls or plates. Drizzle with the reserved topping sauce, sprinkle with toasted sesame seeds or mung beans, and add fresh mango slices if desired. Serve warm.