1. Place the sticky rice in a bowl, cover with plenty of water and set aside to soak overnight.
2. 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 a clean tea towel. Drain the sticky rice and pile it onto the tea towel in the steamer base. Cover with another tea towel and top with the steamer lid. Place in the wok above the simmering water and steam for about 20 minutes.
3. Heat the coconut milk and sugar in a small saucepan over medium heat for about 5 minutes or until the sugar has dissolved. Set aside for later.
4. Once the sticky rice is cooked, place it in a large bowl and stir through most of the coconut milk mixture (reserving about a ¼ of a cup of the liquid). Let the rice sit for 10 minutes.
5. Place the coconut mixture back onto a low heat and whisk through salt and rice flour. Stir over heat until thickened.
6. To serve, divide mango slices between 6 serving plates. Place a spoonful of sticky rice on the side and top with thickened coconut mixture.