Step 1: Rinse the glutinous rice thoroughly under cold running water until the water runs clear. Soak the rice in 1.5 cups of cold water for at least 4 hours, or preferably overnight.
Step 2: Drain the soaked rice completely. Prepare your steamer by lining a bamboo steamer basket with parchment paper or a clean cheesecloth. Place the drained rice evenly in the lined steamer basket.
Step 3: Steam the rice for 25-30 minutes over simmering water, or until it's tender and translucent. Stir gently halfway through for even cooking.
Step 4: While the rice is steaming, prepare the sweet coconut sauce. In a medium saucepan, combine 1 can of full-fat coconut milk (reserve 1/4 cup for garnish later), granulated sugar, and salt. Heat over medium-low heat, stirring constantly, until the sugar is fully dissolved. Do not boil. Remove from heat.
Step 5: Once the sticky rice is cooked, immediately transfer it to a large bowl. Pour 3/4 of the sweet coconut sauce (about 1 cup) over the hot rice. Gently fold the sauce into the rice until it's evenly distributed. Cover the bowl tightly and let it sit for at least 20-30 minutes for the rice to absorb the sauce.
Step 6: While the rice rests, prepare the mangoes. Wash, peel, and slice the ripe mangoes. For best presentation, slice them thinly and fan them out.
Step 7: To serve, scoop a generous portion of the warm sweet sticky rice onto plates. Arrange the sliced fresh mango next to it. Drizzle the reserved 1/4 cup of coconut milk over both the rice and the mango. Garnish with toasted sesame seeds, if desired, and serve immediately.