Place the glutinous sticky rice in a bowl, cover with plenty of water and set aside to soak overnight.
To cook the sticky rice, fill a wok about one-quarter full with water and bring to a simmer. Line the base of a large bamboo steamer with cheesecloth. Drain the sticky rice and pile it onto the cheesecloth and cover with edges of cheesecloth. Cover with bamboo steamer lid. Steam for 20-30 minutes.
In a separate large bowl, make the rice soak. Whisk the coconut milk, sugar and salt.
Once the sticky rice is cooked, place it immediately into the rice soak mixture (while the rice is still hot). Stir until well combined. Cover and rest for at least 45 minutes.
For the salted coconut sauce, combine coconut milk, sugar and salt in a saucepan. Place rice flour in a small bowl and add a little of the coconut milk mixture and stir to avoid lumps. Add into the remaining coconut milk. Cook over medium heat for 3-4 minutes until thickened slightly.
To serve, place a mound of sticky rice on serving plates. Top with mango slices and drizzle with the salted coconut sauce.