Cover the rice with water to a few inches deep and let sit for an hour or overnight. Drain and put in a steamer or pressure cooker, steaming for about 20 minutes or cooking on high pressure for about 10 minutes, until soft but still chewy.
Stir the coconut milk to combine, then heat half of it in a small saucepan to a simmer. Whisk in half the sugar and a large pinch of salt. Pour over the rice in a large, heat proof bowl and stir well to combine. Let sit for about 20 minutes until all the liquid is absorbed.
Heat the remaining coconut milk to a simmer, then use a few spoonfuls to make a slurry with the cornstarch. Whisk this into the coconut milk with remaining sugar and another large pinch of salt, stirring and simmering a few minutes until thickened.
Heap sticky rice on a platter or plates, arrange peeled and sliced mangoes around it, and drizzle with the coconut cream. Sprinkle with sesame seeds and serve immediately.