Place the Thai sweet sticky rice in a large bowl and cover it with several inches of cold water. Let it soak for at least one hour, although soaking it overnight will produce an even softer and chewier texture. Once the soaking time is complete, drain the rice thoroughly before steaming.

Line a steamer basket with cheesecloth or a clean kitchen towel and spread the soaked rice into an even layer. Steam the rice over boiling water for about 20 minutes, or until it becomes tender, translucent, and pleasantly chewy. Unlike regular rice, sticky rice is steamed rather than boiled to achieve its signature texture.

While the rice is steaming, pour half of the coconut milk into a small saucepan and heat it over medium heat. Stir in the sugar and a generous pinch of salt until everything is completely dissolved. The mixture should taste rich, lightly salty, and pleasantly sweet, creating the perfect flavor balance that makes this dessert so irresistible.

Transfer the freshly steamed rice into a large heatproof bowl and immediately pour the warm coconut mixture over it. Stir gently until every grain is coated, then cover the bowl and let it rest for about 20 minutes. During this time, the rice gradually absorbs all the coconut milk, becoming soft, glossy, and infused with creamy coconut flavor.

In a small bowl, whisk the cornstarch with a few tablespoons of the remaining coconut milk until smooth. Heat the rest of the coconut milk in a saucepan, then slowly whisk in the cornstarch mixture. Continue stirring until the sauce thickens into a silky consistency. Finish by adding the remaining sugar and another pinch of salt, then keep the sauce warm until serving.

Spoon generous portions of the coconut sticky rice onto serving plates and arrange slices of ripe mango alongside. Drizzle the warm coconut sauce over the rice and garnish with toasted sesame seeds for added texture and flavor. Serve immediately while the rice is still warm and the mango is fresh and juicy for the best taste and presentation.