Rinse the glutinous rice thoroughly until the water is clear. Place the rice in a bowl and cover it with water, at least 7.5 cm above the rice. Let it soak for 6 hours.
Drain the rice and place it evenly in a steamer lined with parchment paper. Steam for 30 to 40 minutes until tender.
In a saucepan, bring 250 ml of coconut milk to a boil over medium heat, stirring gently. Add sugar and salt, stirring until fully dissolved. Remove from heat once boiling.
In a mixing bowl, combine the steamed rice with the coconut milk mixture, covering it to prevent drying.
Peel and slice the mangoes.
In another saucepan, combine the remaining coconut milk, salt, and cornstarch. Heat gently until thickened.
Serve warm rice with mango slices, top with the thickened coconut sauce, and sprinkle with sesame seeds.