Rinse the glutinous (sticky) rice under cold running water in a fine sieve until the water runs clear.
Transfer the rice to a large bowl, cover with plenty of cold water and leave to soak for at least 4 hours, or preferably overnight.
Drain the soaked rice. Line a steamer basket with muslin or parchment paper, then spread the rice evenly. Steam over gently simmering water (ensuring the water doesn’t touch the rice) for 25–30 minutes, or until it’s tender and translucent. Alternatively, place the drained rice with 360 ml water in a rice cooker and cook on the “white rice” setting.
While the rice is cooking, measure out 240 ml of the coconut milk. Pour this into a medium saucepan with the caster sugar and salt. Warm gently over a low heat, stirring constantly until the sugar has dissolved and the mixture is hot – do not let it boil. Remove from the heat and set aside the remaining 160 ml of coconut milk for serving.
Transfer the hot, steamed rice to a large bowl. Pour about three-quarters of the warm coconut sauce over the rice and gently fold to combine. Cover and leave to rest for 15–20 minutes, allowing the rice to absorb the sauce.
Peel the ripe mangoes and slice the flesh away from the stones into thin slices or cubes.
To serve, pile a portion of the warm sticky rice onto a plate or into a bowl alongside the mango slices. Drizzle over the reserved coconut milk and sprinkle with the toasted sesame seeds.