Rinse rice in a strainer until water runs clear.
In an Instapot or heavy bottom sauce pot combine 1 cup of the coconut milk, water, sugar and salt. Stir well to combine.
If using the Instapot, put lid on and set to Rice setting. When done, release pressure and let stand 10 minutes with the lid on. If using a sauce pot, cover and bring to a simmer over medium low heat 20 minutes or until all liquid is absorbed. Let stand covered for 10 minutes.
While rice is standing, bring remaining 1/3 cup coconut milk and 3 tablespoons sugar to a simmer until sugar is dissolved. Keep sauce warm.
Remove rice from pot and stir in diced mango.
Serve with coconut sauce and garnish with black sesame seeds.