Gather the ingredients.
In a medium to large microwave-safe bowl, pour in sticky rice. Fill the bowl with water and run your hand through the rice to wash it. Pour out water and repeat two more times or until the water runs clear.
Drain the water from the rice thoroughly using a colander. Next, pour the boiling water over the rice and let it sit uncovered for 10 minutes.
While you wait for the rice to soak, prepare the coconut mixture. In a saucepan, combine 1¼ cup coconut milk (leave the remaining ½ cup coconut milk for the coconut sauce) with ¼ cup sugar. Heat over medium heat until sugar is dissolved. Turn off the stove and set it aside.
After 10 minutes of letting the rice soak in boiling water, give the rice a stir. Cover the bowl with a microwave-safe plate (or very loosely with plastic wrap so steam can escape). Place in the microwave and heat on high for 3 minutes.
After 3 minutes, carefully take out the bowl and uncover (there will be lots of hot steam, so be careful). Give the rice a stir from the bottom up. Cover again and heat in the microwave for another 3 minutes.
When finished heating, carefully remove the bowl from the microwave, uncover, and stir the rice. Most of the water should be absorbed at this point. If there is still water left after stirring and letting the steam escape, microwave for another 3 minutes.
Pour in the reserved coconut mixture over the rice. Stir it up and set it aside so the rice can absorb the coconut milk.
Prepare the coconut sauce. In the same saucepan, pour in the remaining ½ cup of coconut milk, 3 tablespoons of sugar, salt, and corn starch. Whisk together. Next, turn heat onto medium, and let the mixture come to a boil and thicken.
Now it's time to serve. Place the sticky rice on a serving dish and arrange the sliced mango next to the rice. Sprinkle sesame seeds (optional) on top and spoon over the coconut sauce. Enjoy!