Rinse and soak the rice. In a bowl, rinse the glutinous rice under cold water until the water runs clear. Cover with fresh water and soak for at least 4 hours, or overnight for best results.
Steam the rice. Drain the soaked rice and transfer to a steamer lined with cheesecloth or a bamboo basket. Steam over simmering water for 20–25 minutes until tender and translucent.
Prepare the sweet coconut sauce. While rice cooks, combine coconut milk, sugar, and salt in a small saucepan. Warm over low heat, stirring until sugar dissolves—do not boil. Reserve about ¼ cup for drizzling.
Mix rice and sauce. Transfer hot steamed rice to a bowl, pour the majority of the coconut sauce over it, and gently fold to coat evenly. Cover and let it rest for 10 minutes to absorb the flavors.
Plate and garnish. Divide the coconut-infused sticky rice onto serving plates. Arrange fresh mango slices on the side, drizzle with reserved coconut sauce, and sprinkle toasted sesame seeds or mung beans for crunch.
Serve warm or at room temperature. Enjoy a mouthful of sweet, creamy mango sticky rice bliss!