Place the rice in a large basin and cover with water. Leave to steep for at least 1 hour. Rinse well and drain, then place in the centre of a clean cloth.
Heat some water in the bottom compartment of a steamer. Place the cloth containing the rice in the upper compartment. Take care to spread the rice out in an even layer. Cover and cook for around 10 minutes.
Open the cloth, wait 1 minute for the rice to cool, then place it in a shallow serving dish, fluffing it up well.
Place the brown sugar, coconut milk and warm water in a saucepan. Blend well and bring to simmering point. Remove from the heat.
Pour the coconut milk syrup over the rice. Leave to cool. Just before serving, add the sliced mango.