Wash rice 3–4 times, then soak overnight.
Drain and steam rice 20–25 minutes until chewy and translucent.
In pan, heat coconut milk, sugar, and salt until boiling.
Stir in ube if using. Mix in rice and cook 4 mins until thickened.
Cool rice for 5 minutes.
Make coconut sauce by heating coconut milk and sugar.
Add corn starch slurry and stir until thick. Set aside.
Slice mangoes into strips.
Assemble rice and mango, drizzle with sauce. Serve fresh.
