Rinse the glutinous rice thoroughly and drain the water. Add the alum and gently rub it through the rice with your hands.

Add water to cover the rice and let it soak for about 4 hours. Drain the water, rinse the rice again, and let it drain for a few minutes.

Steam the glutinous rice for about 20 minutes, or until fully cooked.

In a saucepan, combine the coconut milk, sugar, and salt. Add the pandan leaves and knead them in the mixture until the sugar dissolves. Heat over low heat just until the coconut milk starts to gently simmer (do not let it boil or separate). Remove from heat and set aside.

Once the rice is cooked, transfer it to a container with a tight-fitting lid. Use a spatula to spread out the rice, then immediately pour the prepared coconut milk over it. Stir to combine, cover tightly, and let it sit for about 10 minutes. Uncover and stir again to help the rice absorb the coconut milk. Cover and let it sit for another 10 minutes. Repeat this process until the coconut milk is fully absorbed and the rice is plump.

Peel and slice the mangoes as desired. Serve the sticky rice with the sliced mango. Enjoy!