In a medium size saucepan, combine rice and water and bring to a boil on medium-high heat. Lower the heat, cover the pan and cook till the rice is tender, about 15~18 minutes.
While rice is cooking, combine coconut milk with sugar and salt. Cook until the sugar is dissolved and the mixture is hot.
Pour half the hot coconut milk over hot cooked rice. Cover and set aside for 10~15 minutes.
In a small bowl, whisk cornstarch in 2 tablespoons of water to make a smooth slurry.
Bring the remaining coconut milk mixture to a boil. Then slowly add the cornstarch slurry and cook until it starts to thicken, about 2~3 minutes. Set aside until ready to serve.
Mix the rice with the coconut milk, cover again and set aside to cool completely.
When ready to serve, place the cooled rice on the serving plate. Top it with the chopped mangoes and drizzle the thick coconut milk sauce on top. Sprinkle with sesame seeds (if using) and serve immediately.