Soak the rice: Rinse the rice twice in cold water, then cover it with fresh water and place it in the fridge overnight to soak.
Steam the rice: Drain the rice and transfer it to a piece of cheesecloth. Place the wrapped rice in a steaming basket over boiling water, cover, and steam for 35-40 minutes until the rice is tender but not overcooked. Begin checking for doneness after 30 minutes.
Prepare the sweet coconut sauce: In a saucepan, combine 160ml (⅔ cup) of the coconut milk, 120g (½ cup) sugar, 2g (½ tsp) salt, and the pandan leaf. Bring to a light simmer, stirring until the sugar dissolves, then lower the heat and allow the pandan leaf to steep.
Prepare the second sauce: Add the remaining 100ml (⅓ cup) coconut milk to a small saucepan with the remaining 3g (½ tsp) salt. Warm over medium heat, stirring until the salt dissolves, then remove from heat.
Toast the sesame seeds: In a dry pan, toast the sesame seeds over medium heat until golden, then set aside to cool.
Mix the rice and sauce: Once the rice is cooked, transfer it to a bowl. Gradually add the sweetened coconut milk in thirds, gently folding it into the rice. Add only as much coconut milk as the rice can absorb, being careful not to break the grains. Cover and let the rice rest for 20 minutes.
Serve: Slice the mango cheeks. Place a generous spoonful of sticky rice onto a plate alongside the mango. Sprinkle with toasted sesame seeds, and serve the unsweetened coconut milk on the side to drizzle over just before eating.