Steam the sticky rice until tender (about 25 minutes).
Meanwhile, place the coconut milk in a heavy 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 tender, turn it out into a 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.
Meanwhile, peel mangoes. Remove the seed (see YouTube for further instruction). Slice the flesh thinly crosswise.
To serve, place the remaining milk mixture in a bowl for extra sauce, as individual taste requires. Serve rice and mangoes in presentation or let guests serve themselves.