Mango Sticky Rice is a popular Thai dessert, called Khao Nieaw Mamuang in Thailand. Although not a difficult dish to prepare, sticky rice does require a little patience because it needs time to soak in water. But it's well worth it.
In a bowl wash rice well in several changes of cold water until water is clear. Soak rice overnight in cold water (enough water to cover rice completely).
Drain rice in a sieve. Set sieve over a large deep saucepan of simmering water (sieve should not touch water) and steam the rice, covered with a kitchen towel and a lid, for 30 to 40 minutes, or until tender. Check water level in pan occasionally, adding more water if necessary.
While rice is cooking, in a small saucepan bring 1 cup coconut milk to a boil with 1/3 cup sugar and salt, stirring until sugar is dissolved. Remove pan from heat, and keep mixture warm.
Transfer cooked rice to a bowl and stir in coconut-milk mixture. Cover, and let rice stand for 30 minutes, or until coconut-milk mixture is absorbed. Rice may be prepared up to this point 2 hours ahead and kept covered at room temperature.
While rice is standing, in another small saucepan slowly boil remaining 1/3 cup coconut milk with 3 tablespoons sugar, stirring occasionally, for 1 minute. Transfer sauce to a small bowl and chill until cooled and slightly thickened.
To serve, divide the rice into 6 potions. Scoop rice into small ramekins (or measuring cups), and invert onto dessert plates, forming a rice mound. Drizzle sauce over the rice mounds, and sprinkle coconut on top and garnish with mint (if using). Arrange mango slices among the plates, and serve.