Rinse the glutinous rice thoroughly under cold running water in a fine-mesh sieve or a bowl, swirling with your hands, until the water runs mostly clear. This usually takes 5-7 rinses.

Place the rinsed rice in a large bowl, cover with at least 2-3 inches of fresh cold water, and let it soak for a minimum of 4 hours, or ideally overnight (8-12 hours). This step is crucial for achieving the correct texture.

Drain the soaked rice well. If using a steamer, line your steamer basket with cheesecloth or a clean kitchen towel. Spread the rice evenly in the basket. Steam over vigorously boiling water for 20-25 minutes, or until the rice is tender, translucent, and sticky. If using a rice cooker, follow your appliance's instructions for glutinous rice, but ensure the rice is well-drained first.

While the rice is steaming, prepare the sweet coconut milk sauce. In a medium saucepan, combine 1 cup of the full-fat coconut milk, granulated sugar, and salt.

Heat over medium-low heat, stirring constantly with a whisk until the sugar and salt are fully dissolved. Bring it to a gentle simmer, then remove from heat immediately. Do not boil vigorously, as it can separate.

Once the rice is cooked, transfer it to a large bowl. Immediately pour about 3/4 cup of the warm sweet coconut milk sauce over the hot rice. Stir gently with a spatula or spoon to coat all the grains. Cover the bowl with a lid or plastic wrap and let it sit for 15-20 minutes to allow the rice to fully absorb the coconut milk.

While the rice is absorbing, peel and slice the ripe mangoes. For presentation, you can make fan-like slices or simple wedges. Do this just before serving to keep the mango fresh.

To serve, scoop a portion of the warm sticky rice onto a plate. Arrange the fresh mango slices alongside the rice. Drizzle the remaining 1/2 cup of plain full-fat coconut milk (or any leftover sweet coconut milk sauce) over the mango and rice for extra richness.

Garnish with toasted sesame seeds or fried mung beans for a slight crunch. Serve immediately.