Put the rice in a saucepan with the coconut milk, salt and sugar. Bring to the boil and simmer, stirring occasionally with a wooden spoon, until the rice has absorbed all the liquid and has become very soft. Transfer to a steamer and steam the rice for 15 minutes.
(While the rice is steaming, start making the sankhaya, if you intend to include it.
Let the rice cool somewhat, then mould it. One way is to use individual small moulds, preferably lined with clingfilm/plastic wrap or aluminium foil. Fill each mould, press down fairly firmly with the back of a spoon, then turn the rice out on to the middle of a dessert plate and arrange slices of mango around it.
Alternatively, spread the rice on a tray lined with aluminium foil, then roll it flat with a wetted rolling pin and cut it into diamond shapes with a sharp, wet knife. Serve as above.