Drain the rice to removed the water and place in a hot steamer basket lined with cotton cloth. Make sure the basket is sitting over a few inches of boiling water. Steam the sticky rice for 25 minutes until the rice looks shiny, translucent, and soft and chewy to the texture.
Meanwhile heat the coconut milk with sugar and salt in a pot until hot and the sugar dissolves.
When the rice is done, transfer to a large bowl and pour about 2/3 of hot coconut milk over the rice and stir. Cover the rice with cotton cloth and let it sit to soak up the milk for 20 minutes or longer.
Peel mangoes and slice each side off the seeded are in the center. Mango seeds are flat and if your knife is hitting the seed, move the knife a little off from the seed and slice down. Lay the mango slice flat and cut into big chunks.
To serve, place a mound of rice on a plate and lay a few slices of mango on the top of the rice or on the side. Drizzle the remaining coconut milk over and sprinkle with a little bit of sesame seeds, if using.