Rinse rice very well. Soak rice overnight or for at least 8 hours. Drain and rinse the rice. Place rice in a steamer pot with parchment paper underneath or on a thin cheesecloth. Steam for 20-25 minutes on medium heat. Until rice is translucent and still chewy.
Make the coconut milk sauce. Add coconut milk, sugar, and salt to a small pot over medium heat. Stir frequently until it comes to a light boil. Turn heat to low and continue stirring until sugar is dissolved. Take out 3/4 cup of the the coconut sauce and set aside.
Mix the cornstarch and water together in a small bowl and add it to remaining sauce in the pot. Bring to a simmer and simmer until sauce thickens a little bit, about 2-3 minutes. It’s not a super thick sauce, just thick enough to coat the back of a spoon. This will be used for the topping later.
Transfer rice to a bowl. Pour the reserved 3/4 cup of the coconut sauce over the rice and mix until evenly combined. Loosely cover with a towel and let it sit for 15 minutes for the rice to absorb the coconut milk.
Make the garnish by combining the crushed peanuts, sesame seeds, sugar, and salt in a small bowl.
To serve, scoop some rice onto a plate and add sliced mangoes on the side. Drizzle thickened coconut sauce over the rice and sprinkle some of the garnish on top. Enjoy warm! Best when eaten fresh.