Rinse the sticky rice under cool water until the water runs nearly clear, about 4 to 5 changes. Cover with cool water by 2 inches and soak at room temperature for at least 4 hours or overnight.

Drain the rice. Line a metal steamer basket with a double layer of cheesecloth, spread the rice in an even layer no more than 2 inches deep, and fold the overhang loosely over the top.

Bring 2 inches of water to a boil in a large pot, then reduce to a steady simmer. Set the basket in the pot, cover tightly, and steam for 20 minutes. Flip the rice bundle, cover, and steam another 10 to 15 minutes until the rice is translucent, tender, and slightly chewy. Keep covered off the heat.

Combine 1⅓ cups coconut cream, ¼ cup palm sugar, and ½ teaspoon salt in a medium saucepan. Heat over medium, stirring constantly, until the sugar dissolves and the mixture just begins to bubble and thicken, 5 to 6 minutes. Remove from heat as soon as it thickens.

Transfer the hot rice to a large heatproof bowl and pour about three-quarters of the warm sauce over it. Fold gently 2 to 3 times, cover tightly, and let stand undisturbed for 20 to 25 minutes until the rice absorbs the sauce.

Meanwhile, combine the remaining ⅓ cup coconut cream, 2 teaspoons palm sugar, and ¼ teaspoon salt in the same saucepan. If using rice flour, whisk it into the cold cream first. Heat over medium-low until the sauce coats a spoon, 3 to 4 minutes.

Plate the warm rice with sliced mango alongside, drizzle the reserved sauce over the rice, and top with crushed peanuts and sesame seeds if using.