Soak the glutinous rice in water for at least 2 hours, or overnight if possible. Drain thoroughly.
Steam the rice in a steamer or over a pot of boiling water using a makeshift aluminum foil ball setup for 20-30 minutes. Check at the 20-minute mark; the rice should be tender but not mushy.
While the rice cooks, prepare the mangoes by slicing them into bite-sized pieces.
For the coconut sauce, in a small pot, bring 1 cup of coconut milk, sugar, and salt to a boil. Reduce the heat and simmer until the sugar dissolves.
Mix 1 tablespoon of coconut milk with cornstarch and add to the pot, stirring continuously until the sauce thickens slightly.
Once the rice is cooked, mix in half of the coconut sauce, allowing it to soak in and cool for about 15-20 minutes.
To serve, spoon the sticky rice onto plates, arrange the mango slices around the rice, and drizzle with the thickened coconut sauce. Garnish with sesame seeds.