Soak the sweet rice in 1 cup water for 20 minutes, or up to 1 hour.
The sticky rice can be steamed or made in a pot. To make it in a pot, do not drain. Add 3/4 cup (more) water to the rice, plus 1/4 can coconut milk, 1/4 tsp. salt, 1 tsp. coconut flavoring, and 1 Tbsp. brown sugar. Stir this into the rice, lifting any rice grains that have stuck to the bottom of the pot.
Bring to a gentle boil, then partially cover with a lid (leaving some room for steam to escape). Turn the heat down to medium-low (#2.5 to 3 on the dial).
Allow to simmer for 20 minutes, or until the water has been absorbed by the rice. Remove the pot from the heat, place the lid on tight, and leave to steam cook for 5-10 minutes.
Make sauce and serve the dessert right away, or store the rice (covered) in the refrigerator until you're ready to use it.
To make the sauce, warm the rest of the can of coconut milk together with 1/4 cup sugar, a pinch of salt, 1 tsp. coconut flavoring (optional) and 1 tsp. vanilla flavoring over medium heat (5 minutes).
Add cornstarch (dissolved in the water) to the sauce and stir to thicken it slightly. As it thickens, turn down heat to low. When thickened, remove from heat. Tip: Try not to boil the sauce, or you will lose that wonderful coconut flavor.
Before serving, taste-test the sauce for sweetness, adding more sugar if desired. If too sweet for your taste, add a little more coconut milk.
There are 2 ways to serve this dessert:
Place scoops of the sticky rice in bowls (Note that sticky rice can be served cold, warm, or at room temperature, as desired). Ladle a generous amount of warm coconut sauce over the rice (creating an island of sticky rice). Add slices/pieces of ripe fresh mango. (If using frozen mango, be sure to give it enough time to thaw before serving.)
For saucier rice: Add rice (portion out 1 scoop per person) directly to the sauce pot and stir over low heat, gently breaking apart the larger lumps, but leaving smaller lumps/chunks). Now add the mango pieces and gently stir until everything is warmed through. Portion out into serving bowls, making sure everyone has equal amounts of rice, mango, and sauce.