Drain the rice and place it in a hot wok or saucepan. Stir in the coconut milk, sugar and salt. Bring to a simmer and cook until the coconut milk has been absorbed and the rice is thick and sticky, 4 to 6 minutes. Stir in the oil, then transfer the mixture to a baking sheet and cool for 5 to 10 minutes.
Soften the banana leaves by holding them over a gas fire, heating them in a hot frying pan, steaming or microwaving them for a few minutes. Lay down 1 banana leaf. Place one heaped tablespoon of the cooled rice in the center. Add a single piece of banana, followed by another tablespoon of rice (the banana should be sandwiched). Now fold the sides of the leaves over, as you would a parcel. Repeat with the remaining rice, banana and banana leaves. If you have softened the leaves properly, you won't need to use anything extra to secure the parcels.
Fill a large steamer, wok or saucepan halfway with water and bring to a rapid boil over high heat. Place the parcels in a steamer basket seam-side down and set it over the pan of water. Steam for 30 minutes. Enjoy the sticky rice cakes hot.