1. In a bowl, soak sticky rice with water (add water until rice is covered, about 2 inches of water above rice). Add alum and mix well. Leave the rice soaked for 3-4 hours or overnight.
2. After the rice is well soaked, wash it thoroughly and rinse.
3. In the steamer, put water in the bottom and cover the steam section with cheese cloth. Pour the sticky rice on the cheesecloth, cover with the lid and put it on the stove on medium to high heat for 20 minutes.
4. While waiting for sticky rice, take coconut milk, sugar, and salt to boil on medium to low heat. Stir often till coconut milk is hot. Turn off the heat.
5. When the sticky rice is cooked, quickly remove and put in a container (a bowl or a deep pie plate), spread them evenly. Then, pour the coconut milk into the hot sticky rice immediately. Mix coconut milk with the hot sticky rice. Cover the container with a plate and let it sit for about 5 minutes or until the rice absorbs all the coconut milk.
6. Cut banana leaves in round shape, place sticky rice and sliced mango on for serving.