Place the rice in a large bowl and cover it with water, swirling the rice in your hand so the water gets cloudy. Empty the water then refill it. Wash the rice 6-7 times or until the water runs clear.
Add the rinsed and drained rice to a medium pot along with 1 ½ cups of water. Bring the water to a boil. Once boiling, reduce to a simmer, cover and allow the rice to cook for 15 minutes. Once 15 minutes has passed, remove the pot from the heat and allow it to sit, covered, for another 5 minutes.
When the rice is almost done cooking, add the coconut milk, sugar, and salt together in a separate, small pot over medium heat. Stir until the sugar dissolves.
When the rice and coconut milk mixture have each finished cooking, dump the rice into a large bowl. Drizzle about 1¼ cup of the coconut milk mixture onto the rice ¼ cup at a time, mixing between each addition. The rice should take on a pudding-like texture, so feel free to add more or less as needed to get this texture.
Allow the sticky rice to cool to room temperature (you can also serve warm, if desired).
While it is cooling, make the coconut sauce topping. To do so, add 1 cup of coconut milk, ½ cup granulated sugar, and ½ tsp salt into a small pot. Bring it to a simmer.
Once simmering, mix the cornstarch with 1-2 tbsp of cold water in a small bowl to make a slurry. Add the slurry into the warm milk and mix again until the coconut milk thickens.
To serve, place a mound of sticky rice on a plate or bowl, and drizzle it with the thickened coconut milk mixture and sprinkle sesame seeds on top.
Place ½ of a mango (peeled and diced/sliced) next to the mound of rice. Serve and enjoy!