Rinse 1 cup of Thai white glutinous rice until the water runs mostly clear. Cover with fresh water; soak for at least 4 hours.
Drain well. Place in a damp cloth-lined steamer, leaving room around the rice for steam. Cover and steam over boiling water for 20-25 minutes, until tender.
Meanwhile, heat ⅔ cup of coconut milk, ½ cup of sugar, and ½ teaspoon of salt just until bubbling. Cover and keep hot.
Immediately mix the hot rice with the hot coconut syrup. Cover for 20 minutes, fold gently, then cover for another 20 minutes.
For the sauce, mix 1 teaspoon of rice flour and 1 tablespoon of water. Add ½ cup of coconut milk for the sauce and ¼ teaspoon of salt for the sauce. Bring to a boil, stirring; remove from the heat and cool.
Peel and slice 3 ripe mangoes. Serve alongside the rice with the coconut sauce.
