Wash the glutinous rice until the water runs clear. Soak in water overnight or at least 6 hours.
In a colander or steamer basket, line with cheesecloth or a thin tea towel. Drain the rice and transfer the rice onto the cheesecloth.
Even out the rice and leave some space along the edges to allow steam to flow. Poke several holes in the rice with your finger to allow even cooking. Fold the edges of the cheesecloth over the rice.
Cover and steam the rice for 30 minutes or until tender. The rice should appear slightly translucent and tender all the way through.
Make the coconut sauce 5 minutes before the rice is finished cooking. In a small saucepan, heat the coconut milk, sugar and salt over medium heat. Stir continuously until the sugar and salt dissolves. Remove from heat and cover to keep warm.
When the rice is done steaming, transfer it to a large mixing bowl. Add in ⅔ of the coconut sauce. Reserve the rest for later. Mix well with a silicone spatula and work out any lumps.
Cover and let the rice rest for 30 minutes to allow it to soak up the sauce. Give the rice another stir to redistribute the sauce that has settled to the bottom. Cover and let it rest for another 15 minutes.
In the meantime, peel the mangos. Slice and keep in the refrigerator.
To serve, place half a sliced mango on a plate with a side of sticky rice. Drizzle the reserved coconut sauce over the sticky rice and top with sesame seeds.