Cover rice with several inches of fresh water in a microwave-safe bowl; let stand for 30 minutes. Drain off water so rice is covered by 1/4 inch of water.

Microwave rice, covered, on High until water is mostly absorbed but rice is still wet, about 10 minutes; stir and cook until almost dry, 4 minutes more.

Combine 1/2 coconut milk and 1 cup sugar in a bowl, stirring until sugar dissolved. Pour over rice; stir until rice coated. Cover and let rice stand at room temperature for 20 minutes.

Pour remaining 1/2 can coconut milk into a saucepan; whisk in remaining 1 tablespoon sugar, cornstarch, and salt until smooth. Bring mixture to a simmer over medium heat, whisking constantly; simmer until thickened, about 2 minutes. Remove from heat; cool.

Scoop rice into individual serving bowls; top each serving with about 2 tablespoons coconut sauce and several mango pieces.