Steam the sticky rice until soft.
Meanwhile, place the coconut milk in a pot and heat over medium heat until hot. Do not boil. Add the sugar and salt and stir to dissolve completely.
When the sticky rice is cooked, turn it out into a large bowl and pour 1 cup of the hot coconut milk over; reserve the rest. Stir to mix the liquid into the rice, then let stand for 20 minutes to an hour to allow the flavors to blend.
Peel and chop the mangoes.
If you want to serve individually, place a round mound of rice on a plate and top with chopped mango.
Stir the remaining sweetened coconut milk thoroughly, transfer to a small serving bowl and pass it separately, with a spoon, so people can spoon on extra.