Rinse the Sticky Rice: Place the black sticky rice in a bowl and rinse with water until the water runs clear. Place the white sticky rice in a separate bowl and rinse with water until the water runs clear. Pour out the water from both bowls.
Cook the Black Sticky Rice: Add the black sticky rice to a pot with water. Bring to a boil. Once the water comes to a boil, immediately turn it off and take the pot off the heat. Allow to cool completely.
Soak the Sticky Rice: Add the cooled black sticky rice (with the water) to the white sticky rice. Allow to soak for at least 4 hours and up to overnight.
Steam the Sticky Rice: Pour the soaking water off of the sticky rice. Then place the sticky rice into the bamboo steamer basket on top of an aluminum pot filled with boiling water. Steam the sticky rice until the rice grains are soft and translucent (approximately 40 minutes).1 (If you are new to making sticky rice, please read the detailed directions in this HOW TO: Make Thai Sticky Rice).
Make the Coconut Sauce: While the rice is steaming, mix the coconut milk, salt, and sugar over medium heat until the sugar and salt are dissolved and the mixture is smooth.
Mix Rice with Cream: Once the sticky rice is done, add the sweetened coconut milk sauce and mix together with the rice. Cover the rice and allow to sit for 20 minutes until the coconut milk has been absorbed into the rice.
Make Thick Coconut Sauce: Make the thickened coconut sauce by combining the coconut milk, sugar, salt, and rice flour (or cornstarch) in a small pan over medium heat. Stir and allow to cook until the mixture becomes slightly thick but still pourable.
Serve: Place a scoop of coconut-flavored sticky rice on a plate. Slice a ripe mango and arrange half of it next to the sticky rice. Spoon the thickened coconut sauce on top and sprinkle with crispy mung beans. Enjoy!