Rice the rice well with cold water. Place the rice and 3 cups of water in a large pot over medium-high heat and bring to a boil. Reduce the heat to low, cover, and continue to cook for about 25 minutes, or until all the water has been absorbed.
While rice is cooking, in a small saucepan bring 1 cup coconut milk to a boil with 3 tablespoons agave and the salt. Once it’s reached a boil, remove from the heat and cover to keep it warm.
Place the cooked rice in a heat-proof bowl and stir in coconut milk mixture. Cover and let sit for 30 minutes, or until the coconut milk mixture is absorbed.
While the rice is sitting, make the sauce by whisking together the remaining 1/3 cup coconut milk and 1½ tablespoons agave in a small saucepan, and bring to a simmer. In a small bowl, mix together the cornstarch and the remaining 1 tablespoon of water. Add the cornstarch slurry to the pan and continue to whisk until the mixture thickens. If it becomes too thick, whisk in a little water to bring it to the desired consistency.
To serve, place a scoop of rice in a small bowl or plate along with some of the mango, then drizzle on the coconut sauce and top with the toasted sesame seeds.