In a large bowl, wash rice until water is almost clear. Soak rice in water overnight, 12 to 24 hours.
To make the coconut cream sauce: In a small bowl, combine 2/3 cup coconut milk and 3 Tablespoons sugar. Bring to a boil and then reduce heat and simmer until thickened to desired consistency, stirring occasionally. Remove from heat and set aside.
Line a colander with cheesecloth. Pour in rice to drain. In a large both with a steamer insert, bring water to a boil.
Wrap the rice in the cheesecloth and place into the steamer insert, distributing the rice evenly. Steam the rice for 15 to 20 minutes or until the grains become translucent.
While rice is steaming, combine the remaining coconut milk, 6 Tablespoons sugar and salt in a small saucepan. Heat the mixture over medium heat, stirring well until the sugar is dissolved. Do not boil, but keep warm.
When rice is cooked, transfer to a large bowl and add warm coconut milk. Stir to combine. Cover bowl with a dish cloth and let sit for at least 5 minutes or until the rice absorbs all the coconut milk.
In the meantime, peel and thinly slice the mangoes.
Place a scoop of stick rice on a small serving plate (a 1/2 cup measuring cup makes a nice mold), garnish with toasted sesame seeds and serve with sliced mango drizzled with coconut cream sauce.