Soak the rice: Rinse the glutinous rice until the water runs clear, then soak in a bowl of water for at least 8 hours or overnight.

Steam the rice: Drain and place in a bamboo steamer lined with parchment paper. Steam for 20-25 minutes until tender.

Prepare the coconut mixture: In a saucepan, heat the coconut milk, sugar, and sea salt until the sugar dissolves. Add ube if using.

Combine rice and coconut: Add the steamed rice to the coconut mixture, stir gently, and simmer on low for about 10 minutes.

Make the coconut sauce: Heat additional coconut milk with sugar, create a cornstarch slurry, and add it to the boiling coconut mixture until thickened.

Assemble the dish: Slice ripe mangoes, serve sticky rice, and drizzle with coconut sauce. Garnish with sesame seeds if desired.