Rinse the sticky rice until the water runs mostly clear. Soak in cold water for 6–8 hours, then drain.
Set up a steamer and line the basket with cheesecloth or a thin towel. Bring the steamer water to a boil.
Steam the drained rice for 20–25 minutes until tender and sticky. Keep hot.
Warm coconut milk, sugar, and salt for the sauce in a saucepan. Stir until dissolved; do not boil.
Transfer hot rice to a bowl. Fold in the warm coconut sauce, cover, and rest 15 minutes to absorb.
Whisk cornstarch with water to make a smooth slurry for the topping.
Simmer remaining coconut milk with sugar and salt. Whisk in the slurry and cook until slightly thick and glossy.
Slice the mango. If needed, lightly toast sesame seeds in a dry skillet until fragrant.
Plate the sticky rice, drizzle with coconut topping, add mango slices, and finish with sesame seeds.