Rinse the sticky rice with tap water, gently stirring it with your hand to remove excess starch. Drain the water and repeat this process once more before covering the rice with cold water to soak. Let it soak for a minimum of one hour, or up to overnight in the refrigerator for best results. After soaking, give it another rinse and drain again.

Transfer the soaked rice into a heatproof bowl and set it in a steamer, ensuring you add about two inches of water to the steamer without getting any into the rice itself. Cover the steamer and bring it to medium-high heat until boiling, then reduce to medium heat. Steam for 20-25 minutes until cooked through, making sure there are no hard bits remaining.

In a saucepan, combine full-fat coconut milk, sugar, and salt over medium heat until heated through and sugar has dissolved completely—just be careful not to bring it to a boil as you want a smooth sauce.

If your coconut milk appears thin, mix cornstarch with four tablespoons of water in a small bowl until dissolved. Add half of this slurry into your sauce while stirring constantly to achieve your desired thickness; adjust further as needed so it's just thick enough to coat the back of a spoon.

Once your sticky rice is done steaming, carefully remove it from the steamer using oven mitts. Pour one cup of your prepared coconut sauce over the warm sticky rice and stir well until fully absorbed for that perfect blend of flavors.

Let both the mango sticky rice mixture and remaining sauce cool down slightly before serving them together at room temperature or lukewarm. Scoop about one-third cup of coconut rice into serving bowls or plates alongside slices of fresh mango and drizzle additional coconut sauce on top as desired, finishing with toasted sesame seeds if you choose!