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.

Meanwhile, in a small saucepan, bring half the coconut milk to a simmer over medium heat, stirring frequently. Whisk in ¼ cup plus 2 tablespoons sugar and a large pinch of salt until dissolved.

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.

Meanwhile, rinse and dry the saucepan. In a small bowl, whisk 2 tablespoons of the remaining coconut milk with the cornstarch until smooth. Add the rest of the remaining coconut milk to the saucepan and bring to a simmer over medium heat, whisking occasionally. Whisk in the slurry, return to a simmer, and cook, whisking constantly, until thickened and glossy, about 2–3 minutes. Whisk in remaining 2 tablespoons sugar and a large pinch of salt until dissolved. Keep coconut mixture warm.

When ready to serve, mound coconut rice onto plates and arrange sliced mango alongside. Drizzle coconut cream all over rice and garnish with toasted sesame seeds. Serve right away.