While the rice is steaming, open the can of coconut milk and spoon out the thick cream on top into a small bowl and set aside. If you do not shake your can and let it chill in the refrigerator before using, this step will be very easy as the milk and cream will naturally separate. Pour the thinner, lighter coconut milk that remains into a small pan.
Add 2 tablespoons of sugar and ½ teaspoon of salt to the coconut milk in the small pan, and warm over medium heat, stirring gently and continuously until the sugar is completely dissolved, about 5 minutes. Be careful not to let the sauce come to a boil.
Once the rice is cooked through with no hard, raw bits in the middle of the grains, remove the splatter guard or strainer from over the water and scrape the rice off the screen into a mixing bowl. It may be lumpy, which is fine. Slowly pour the warm coconut milk mixture over the cooked rice, stirring frequently to coat the rice in the coconut milk mixture, but watching so that you are not oversaturating the rice to the point that there are puddles of coconut milk. Continue stirring, and stop adding the sweetened coconut milk once it looks like the rice is saturated. You may not use all of the coconut milk mixture. Set the rice aside for 20 minutes (or even up to 2 hours) to finish absorbing the coconut milk.
While the rice is soaking, make the coconut cream topping. In the same saucepan that you used for the coconut milk, give it a quick rinse and then stir together the reserved coconut cream and the remaining 1 tablespoon of sugar, and ¼ teaspoon salt. In a separate bowl, whisk together a few teaspoons of water and the cornstarch to create a thin slurry.
Whisk this cornstarch slurry into the coconut cream mixture and cook over low heat for about 3 minutes, or just until the mixture begins to thicken. Set aside.
Peel the mango and slice it into thin slices. Arrange the slices in a decorative manner on individual serving plates and serve with a portion of coconut sticky rice with the coconut cream sauce drizzled over the top and toasted sesame seeds sprinkled on top for garnish. You can also garnish the plate with banana leaves or tropical, if you have access to them, just to make it look even more authentic.