Tip the rice into a sieve and drain well. Put the rice into a pan with enough water to cover the rice by about 1cm|1/2".
Bring to a boil, then turn down the heat to very low, cover the pan with a tight fitting lid and cook for about 20 minutes, until the rice has absorbed all the water. Take the lid off the pan and leave to cool.
Take 2 tablespoons of the thick upper layer of coconut milk and put into a pan. Mix smoothly with the cornflour, salt and 8 tablespoons of water and bring to a boil. Turn the heat down and cook, stirring for 1 minute. Remove from the heat and set aside. This will be the coconut sauce.
Put the remaining coconut milk into a large pan, stir in the sugar and 10 tablespoons of water and stir over a low heat until the sugar has dissolved completely. Bring to a boil, then remove from the heat, add the sticky rice and stir well. Set aside.
Peel the mangoes carefully with a sharp knife, remove the stone and slice the flesh.
Spoon the sticky rice into 4 serving bowls, add the mango slices and pour the coconut sauce over the rice.