Prepare the rice: Wash the sticky rice thoroughly 5-6 times until water runs almost clear, rubbing gently to remove excess starch. Soak in cold water for at least 4-6 hours or overnight.
Steam the rice: Drain the rice very well. Line a steamer basket with cheesecloth or a clean kitchen towel. Add the drained rice and steam over boiling water for 25-30 minutes until tender and translucent. The rice should be sticky but still have distinct grains.
Coconut syrup: While rice is steaming, combine coconut milk, sugar, and salt in a bowl.
Mix: Transfer the hot steamed rice immediately into a large bowl. Pour the coconut syrup over the rice and gently fold together until well combined. Cover and let stand for 30 minutes to 1 hour, allowing the rice to absorb the coconut mixture.
Make the coconut sauce: In a small saucepan, combine coconut milk, sugar, salt, and rice flour. Heat over medium heat, stirring constantly, until it just comes to a boil. Let cool.
Prepare the mangoes: Peel and slice the mangoes. The traditional way is to slice each cheek off the pit, then score the flesh in a crosshatch pattern or slice into strips.
Serve: Place a portion of sticky rice on each plate alongside fresh mango slices. Drizzle with the coconut sauce and sprinkle with toasted sesame seeds or crispy mung beans if desired. Serve at room temperature.