1/2 cup light coconut milk
1/2 cup granulated sugar
1/2 teaspoon salt
1 cup black sticky rice
1/2 cup water
1/4 cup light coconut milk
1/4 cup shredded unsweetened coconut, divided
2 mangos, sliced into lengthwise strips
Whisk 1/2 cup coconut milk, sugar, and salt together in a bowl until sugar dissolves and coconut sauce is smooth.
Combine rice, water, 1/4 cup coconut milk, and 1 tablespoon shredded coconut in a saucepan over medium heat; simmer, stirring frequently, for 10 minutes. Decrease heat to low and cook until rice is sticky and soft, 20 to 25 minutes.
Pour 1/2 of the coconut sauce over rice; stir and fluff with a fork. Add remaining coconut sauce to rice mixture and stir well. Remove saucepan from heat and let stand until most of the liquid is absorbed, about 15 minutes.
Cook and stir remaining shredded coconut in a skillet over medium-low heat until lightly toasted, about 5 minutes.
Ladle about 1/2 cup rice mixture into each serving bowl or plate. Arrange 4 to 5 slices mango on each serving; sprinkle toasted coconut over mango.