For the glutinous rice, add to a large bowl. Rinse and drain with cold tap water for at least 5 times. Cover rice with water by several inches and let sit at room temperature for 4 hours or up to overnight.
Fill your steaming pot or saucepan (that fit well with the steamer) with 2 inches of water. Bring to a boil over high heat.
Prepare a steamer. Line it with a cheesecloth or a clean kitchen towel. Drain the rice and add to the cloth. Cover the rice with the cloth and smooth the rice to form an even layer. Transfer to the steaming pot, cover with a lid and cook for 25 minutes.
Meanwhile, in a small saucepan over medium heat, combine ¾ cup coconut milk, ½ cup sugar, and ½ teaspoon salt. Cook until the sugar is dissolved, stirring often. Turn off the heat. Cover with a lid to keep warm.
When the rice is cooked, carefully transfer the rice to a medium heatproof bowl. Add the warm coconut milk mixture over and mix well. It should look like a lot of liquid at this point. Don't worry. Cover with a piece of aluminum foil. Let it sit for 20 minutes. Stir gently and cover with the foil again. Sit for 10 more minutes (or up to 2 hours).
In the meantime, prepare the mangoes. Cut the mangoes in 3 parts lengthwise. Remove the pit. Carefully scoop out the flesh and slice into thin slices. Set aside.
For the sauce, in a small saucepan over medium heat, add ½ cup coconut milk and ¼ teaspoon salt. Cook until hot. In a small bowl, add corn starch and 1 tablespoon coconut milk. Stir until no lumps. Add to the hot coconut milk, stirring quickly to thicken the sauce, about 1 minute. Transfer to a small bowl.
To assemble, place rice on serving plates, top with some coconut sauce, sprinkle with sesame seeds and place the mangoes next to the rice. Serve immediately.