Rinse the glutinous rice thoroughly under cold running water until the water runs clear. Place the rinsed rice in a large bowl, cover with 4 cups of fresh cold water, and let it soak for at least 4 hours, or preferably overnight, at room temperature. This step is crucial for proper texture.

Drain the soaked rice very well. Line a steamer basket with cheesecloth or a piece of parchment paper (pierced with small holes) to prevent the rice from falling through. Spread the drained rice evenly in the steamer basket.

Fill a large pot with about 2-3 inches of water and bring to a rolling boil. Place the steamer basket over the boiling water, ensuring the water does not touch the rice. Cover tightly with a lid and steam for 20-25 minutes, or until the rice is tender, translucent, and pleasantly chewy. Remove from heat and carefully transfer the steamed rice to a large mixing bowl.

While the rice is steaming, combine the full-fat coconut milk, granulated sugar, and salt in a small saucepan. Heat over medium-low heat, stirring constantly with a whisk, until the sugar and salt are completely dissolved and the sauce is gently simmering (do not let it come to a vigorous boil).

If you prefer a slightly thicker sauce, stir in the cornstarch slurry (1 tsp cornstarch mixed with 1 tbsp cold water) and continue to cook for another minute until the sauce thickens slightly. Remove the saucepan from the heat.

Pour about 1 cup of the hot coconut sauce over the warm steamed rice in the mixing bowl. Stir gently with a wooden spoon or spatula until the rice has absorbed most of the sauce and appears glossy. Cover the bowl tightly with a lid or plastic wrap and let it sit for 15-20 minutes. This resting period allows the rice to fully absorb the sauce, becoming perfectly sticky and infused with flavor.

While the rice is resting, peel the ripe mangoes. Carefully slice the flesh away from the large, flat pit, then cut the mango flesh into attractive slices or cubes.

To serve, scoop individual portions of the warm, sweet sticky rice onto serving plates. Artfully arrange the fresh mango slices alongside the rice. Drizzle generously with the remaining coconut sauce. Garnish with a sprinkle of toasted sesame seeds and a few fresh mint leaves, if desired. Serve immediately and enjoy!