Wash the rice for 5-6 times or till the water gets clear. Soak the rice in water for at least 8 hours or overnight. After 8 hours discard the water in which rice was soaked.
Put the soaked rice on a muslin/ thin cotton towel, cover it and place on a steamer. If you have a traditional bamboo steamer then no need to use the towel. However, since the holes in electric steamer are generally too big for rice grains we need to put the soaked rice in a thin cotton towel before placing it in the steamer. Leave some space near the edges of the steamer so that steam doesn't get blocked by the cloth. Steam the rice for at least 30 minutes or till it's cooked. Make sure to steam the sticky rice instead of boiling.
While the rice is cooking, add coconut milk, sugar and salt in a pan and boil till sugar gets dissolved. Switch off the gas.
In a bowl mix the steamed rice and coconut milk mixture made above. Just ensure to do it when both rice and coconut milk are still hot. At this stage you will feel that there is too much coconut milk and the mixture has become runny but the hot rice will soak it after some time. Cover the bowl with a tight lid and allow to rest for around 15-20 minutes.
After 15-20 minutes, mix everything again so that the rice at the bottom comes up and the coconut milk get absorbed evenly. Cover and allow to rest for another 15-20 minutes.
While the rice is soaking coconut milk, prepare the salted coconut sauce. For that mix salt and coconut milk in a pan and heat it. In a bowl mix the cornstarch and water. Add this cornstarch paste to the coconut milk, mix and allow it to boil for 2-3 minutes or till the mixture thickens.
Peel and cut the mango in desired shape.
To serve, place some sweet coconut rice on a plate, drizzle some salted coconut sauce on top and put a few mango pieces on the side.
Enjoy your Thai Mango sticky rice.