Rinse the glutinous rice under cold water until the water runs mostly clear. Cover with plenty of cold water and soak at least 8 hours or overnight, then drain well.
Line a steamer basket with cheesecloth and spread the drained rice in an even layer. Steam over boiling water 20-25 minutes, flipping once halfway, until translucent and tender with no hard centers.
While the rice steams, warm 1 cup coconut milk in a small saucepan with the sugar, 1/2 teaspoon salt, and pandan leaf until steaming (not boiling) and the sugar dissolves. Remove the pandan leaf.
Transfer the hot rice to a large bowl. Pour over the warm coconut mixture and fold gently with a spatula. Cover and rest 20-30 minutes until the liquid is absorbed.
Warm the remaining coconut milk with the other 1/2 teaspoon salt for the drizzle; thicken with a rice-flour slurry if you like. Keep warm.
Cut the mangoes into cheeks, score in a crosshatch, and press the skin to fan the cubes up. Scoop warm rice onto plates, arrange mango alongside, and drizzle with the salted coconut cream. Sprinkle with sesame seeds if using.