In a large bowl, cover rice with water by several inches and let stand at room temperature 1 hour or up to overnight.
Drain rice. Assemble a steamer, line with cheesecloth or a clean kitchen towel, and add rice, smoothing surface to form an even layer. Set over high heat, bring water in steamer to a boil, cover, and cook until rice is tender, about 20 minutes.
whisk 2 tablespoons of the coconut milk with the cornstarch until smooth.
Meanwhile, in a small saucepan, bring the coconut milk to a simmer over medium heat, stirring frequently. Whisk in ⅓ cup of sugar and a large pinch of salt until dissolved. Add coconut cornstarch mix.
Transfer cooked rice to a large heatproof bowl and pour coconut milk mixture on top. Stir well to combine, cover with plastic, and let stand until liquid is absorbed, about 20 minutes.
When ready to serve, mound coconut rice onto plates and arrange sliced mango alongside. garnish with toasted sesame seeds. Serve right away.
