Add the glutinous rice into a sieve and place it under running water, gently swishing the rice around with your hands.
Place the washed rice into a large bowl and fill with fresh, clean water to cover. Set aside to soak overnight.
The next morning, drain the water from the rice.
Transfer the glutinous rice to a large cheesecloth in an even layer and tuck in cut pandan leaves (if using).
Place a steaming rack into a large pot and fill with enough water, about 2-3" high, making sure that there's enough room between the steamer and the top of the waterline.
Bring steamer to a boil and place in glutinous rice, cover with a lid and steam on medium-high heat for 20 minutes, until rice is soft.
Remove cooked glutinous rice from steamer.
Prepare the coconut milk mixture for the rice:
In a small saucepan, combine 200ml coconut milk with sugar and salt.
Stir and bring mixture to a gentle simmer (not a boil) to warm the coconut milk mixture and dissolve the sugar and salt. Keep warm.
Combine the rice and coconut milk:
Transfer the cooked glutinous rice to a large mixing bowl.
While the glutinous rice is still hot, pour the coconut milk mixture on top of the rice and mix together. (Note: it may look like a lot of liquid, but all of it will be absorbed in about 20 minutes).
The grains of rice will swell and glisten when it fully absorbs the coconut milk. (The texture will resemble a slightly thick, sweet rice pudding).
Cover the coconut sticky rice with a plate or plastic wrap.
Make the sweet coconut sauce:
Combine 150ml coconut milk, sugar, and salt in small saucepan.
Stir and heat the coconut milk gently over the stove, until it comes to a simmer.
In a small bowl, mix together tapioca starch and cold water to form a slurry.
Once the coconut milk comes to a simmer, drizzle in the tapioca starch slurry and stir until the coconut sauce thickens.
Remove thickened coconut sauce from heat and place into a small bowl or pouring vessel.
To serve:
Place a portion of coconut sticky rice on a bowl or plate.
Top with fresh mango slices.
Pour sweet coconut sauce on top of the Mango Sticky Rice and sprinkle toasted sesame seeds prior to serving.