Step 1 Prepare the rice: Soak the rice in water to cover for at least 3 hours, up to overnight. (The longer the rice soaks, the less time it will take to steam.)
Step 2 Drain the rice and transfer to a bamboo steaming basket designed for cooking sticky rice suspended over a pot of boiling water. (Alternatively, steam the rice in a cheesecloth-lined steamer basket or colander suspended over boiling water. Place the rice in an even layer in the steamer basket for even steaming.)
Step 3Wait for the steam to begin rising through the rice, then cover the rice with a damp, thin kitchen towel, folded to completely cover the rice. Gently cover the rice (do not press down the towel) and reduce the heat to maintain a steady flow of steam. Have a pot or kettle of simmering water on the side to add as necessary to maintain the water for steaming. Steam the rice until it swells, glistens and can be molded into small, cohesive balls; the rice will be just tender (be careful not to overcook). Cooking time will vary from about 25 to 40 minutes depending on the cooking vessel and soaking time for the rice.
Step 4While the rice is cooking, combine 2 cups of the coconut milk in a saucepan with the sugar and 2 teaspoons salt. Bring the mixture to a simmer, stirring occasionally, until the sugar is dissolved. Remove from heat and pour the mixture into a heat-proof cup or bowl. Set aside.
Step 5In the same pan, heat the remaining coconut milk to a gentle boil. Add the remaining salt and continue to cook and gently stir for about 3 minutes or until coconut milk thickens slightly. Remove from heat and pour into a heat-proof cup or bowl. Set aside.
Step 6Turn out the steamed rice into a large bowl. Pour the sweetened coconut milk over it and stir gently but well to incorporate thoroughly. Loosely cover the bowl and set aside, stirring occasionally, until the sauce is absorbed by the rice, 30 minutes to an hour. This makes a scant 4 cups rice.
Step 7Serve the rice slightly warm or at room temperature. Mound the sticky rice into each of 6 serving bowls or plates and garnish with sliced mango and toasted tua tong, if using.