Soak 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 teaspoons salt, 1 teaspoons coconut flavoring, and 1 tablespoons 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.
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 are ready to use.
To make the sauce, warm the rest of the can of coconut milk, together with 1/4 cup sugar, and a pinch of salt, 1 teaspoons coconut flavoring (optional) and 1 teaspoons vanilla flavoring over medium heat (5 minutes).
Add cornstarch (dissolved in water) to the sauce and stir to thicken it slightly. As it thickens, turn down heat to low. When thickened, remove from heat (Avoid boiling 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.
Place scoops of the sticky rice in bowls. (Note that tht sticky rice can be served cold, warm or at room temperature, as desired). Ladle a generous portion 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).