Rinse the sticky rice in cold water twice, gently swishing the grains with your fingers each time, and drain. Soak the sticky rice in 500ml water, refrigerated, for at least 4 hours or overnight. Drain when ready to use.
Fill the bottom third of a wok with water and place a bamboo steamer with a lid inside it. Bring the water to a boil over high heat. Place a dampened cheesecloth or towel over the bamboo rack and spread the rice on it, leaving a 2.5cm border all around to let the steam through. Fold over the cheesecloth (or kitchen towel), cover the steamer securely with the lid, and steam until the rice is translucent and tender but firm (about 20 minutes).
Bring the coconut milk to a boil in a small pot over high heat. Reduce the heat to medium low, add the sugar and salt, and stir until both are completely dissolved.
Dissolve the tapioca starch with two tablespoons water and using a whisk, quickly stir it into the coconut milk. Allow the coconut milk sauce to thicken (about one minute). Remove from the heat.
Pour half the coconut mixture into a serving bowl. Transfer the sticky rice to the remaining coconut milk in the small pot and stir to coat the grains evenly. Cover with the lid and let stand until the rice has completely absorbed the coconut milk (about 15 minutes).
Divide the rice among individual plates and spoon a little more coconut sauce over each serving of rice. Then, garnish with an equal amount of sliced mango, sprinkle with sesame seeds.
Serve while still warm or at room temperature.