In a large bowl, combine the rice and enough water to cover by 2-inches. Soak for at least 6 hours or up to 24 hours. Drain.
Inside a wok, place a bamboo steamer and line the steamer with parchment paper. Add enough water to come up just below the bottom steamer rack, 1 to 1 1/2 inches. Bring the water to a boil. Add the rice to the bottom steamer rack, reduce the heat to medium-low, cover, and steam until rice is tender, 20 to 25 minutes. Remove from the heat and transfer the rice to a bowl.
Open the coconut milk and skim the thick cream from the top into a small saucepan. (Reserve the leftover coconut milk for another recipe.) Add the sugar and salt to the coconut cream and bring to a boil. Cook, stirring, until the sugar dissolves, about 1 minute. Pour over the rice, mix well, cover and set aside until liquid is absorbed, about 30 minutes.
Serve at room temperature with mango slices. Garnish with mint sprigs. (The finished dish can be kept, covered, at room temperature for 6 to 8 hours. Don't refrigerate or the rice will harden.)
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