Bring the water to a boil, stir in the drained rice and add 3 tablespoons of the sugar. Boil briefly to dissolve sugar, reduce heat to low. Cover and simmer until water has evaporated, about 8 to 10 minutes. Remove from heat and let sit, covered, for about 20 minutes.
While the rice is cooking, in another small saucepan, combine the coconut milk, salt and remaining sugar. Bring to a low boil. In a small bowl, stir the Rumford Cornstarch with the 1/3 cup water until dissolved. Slowly pour the cornstarch mixture into the simmering coconut sauce. Stir constantly for another 2 minutes until thickened. Remove from the heat and stir in the vanilla.
After the rice has sat for 20 minutes, uncover, fluff with a fork and add about half the coconut sauce to the rice. Use enough to coat the rice, it should be wet but not runny. Set aside for about 20 minutes to absorb the sauce. Reserve the remaining sauce.
To serve, pack the rice in a one-half cup measure and unmold in the center of each plate. Surround with mango slices and divide the reserved sauce evenly over each.