01 - Rinse glutinous rice with cold water several times until the water runs clear. Submerge in water and soak for a minimum of 4 hours or overnight.
02 - Drain the soaked rice. Line a steamer basket with cheesecloth or muslin. Spread the rice evenly in the basket and steam over simmering water for 25 to 30 minutes until tender and glossy.
03 - While the rice steams, combine 1 cup coconut milk, 1/4 cup granulated sugar, and 1/4 teaspoon salt in a saucepan. Warm gently over medium heat, stirring constantly until sugar fully dissolves. Do not allow to boil.
04 - Transfer hot sticky rice to a large mixing bowl. Immediately pour the warm coconut sauce over the rice, stir gently to blend, cover, and let rest for 15 to 20 minutes so rice absorbs the liquid.
05 - In a small saucepan, whisk 1/2 cup coconut milk with cornstarch, 1 tablespoon sugar, and a pinch of salt. Cook over low heat, stirring continuously, until mixture thickens. Remove from heat.
06 - Arrange portions of sticky rice on serving plates. Place mango slices alongside. Drizzle with coconut topping sauce and garnish with toasted sesame seeds or mung beans if desired.