Rinse the sticky rice under tap water, gently stirring with your hand. Drain and repeat once more. Add cold water to cover and soak for at least 1 hour (or up to overnight in the refrigerator). Rinse again and drain.

Transfer rice to a heatproof bowl. Add about 2 inches of water to a steamer (not into the rice). Place the bowl on the steaming rack, cover, and bring water to a boil over medium-high heat.

Once steaming, reduce heat to medium and steam for 20–25 minutes, until the rice is fully cooked and tender. Remove from heat and keep covered.

While the rice cooks, make the coconut sauce. In a saucepan, combine coconut milk, sugar, and salt. Heat over medium or medium-low until hot and the sugar dissolves. Do not boil.

(Optional) If the coconut milk is thin, mix cornstarch with 4 tablespoons water. Add half the slurry to the sauce, stirring immediately to thicken. Adjust as needed—the sauce should coat the back of a spoon.

Carefully remove the rice from the steamer. Pour 1 cup of the coconut sauce over the rice and stir until fully absorbed.

Let the rice and remaining sauce cool to room temperature or lukewarm.

To serve, portion about ⅓ cup coconut rice onto plates or bowls. Add sliced mango on the side, spoon extra sauce over the rice, and garnish with toasted sesame seeds if desired.