MAKE THE SAUCE: In a medium saucepan, combine the coconut milk with the sugar and salt. Bring to a simmer and cook over moderate heat until thickened slightly, about 10 minutes. Transfer the mixture to a sauceboat and let cool to room temperature, about 1 hour.

MEANWHILE, PREPARE THE RICE: In a large, microwave-safe bowl, cover the sticky rice with water; let soak for 1 hour.

Drain the rice and return it to the bowl. Add 2 1/4 cups of water and cover the bowl with plastic wrap. Microwave the rice on high power for 5 minutes. Pull back the plastic wrap and stir the rice. Replace the plastic wrap and microwave the rice for another 5 minutes. Pull back the plastic wrap to stir the rice a second time. Replace the plastic wrap and microwave until the rice is just tender, about 3 minutes longer. Let stand, covered, for 5 minutes.

In a small saucepan, combine the coconut milk with the sugar and salt and bring to a simmer. Pour the hot coconut milk over the rice, folding until just combined. Spoon the rice into bowls. Top with the mango and sesame seeds and serve, passing the sauce at the table.