Unless you've managed to find rinse-free sticky rice (I never have), you'll need to rinse and soak the rice beforehand. Start with the uncooked rice in a large bowl, and add several cups (no need to measure) of cold water. Shake and stir the rice around with your hands; you'll see the water turn white. Drain the water from the rice, and keep repeating this step until the water stops turning completely white. Finally, add water to the bowl to cover the rice and let it soak for at least 6 hours (overnight is best).
If you have a rice cooker, use it to prepare the rice. If not, add the rice and 1 cup of water to a medium pot. Bring the water to a boil, then cover the pot and turn the heat down to medium-low. Let the rice cooked undisturbed for 20 minutes, then check to see if it is done. Cook longer if needed. When the rice is ready, set it aside to cool.
In a large pot, combine the coconut milk (make sure to shake the can well before opening) and sugar. Mix over medium-high heat until the sugar is dissolved and the sauce is combined and creamy.
Add the cooled rice into the pot with the coconut milk mixture. Stir to combine, and continue to cook over medium-high heat until the mixture is the consistency of thick porridge. Remove from heat. You could serve this mixture by itself or with some mango as a dessert rather than continuing with the recipe -- we eat it this way from time to time.
Peel the plantain and slice it into 8-10 rectangular pieces.
Rinse the banana leaves and cut them down to around the size of a standard sheet of paper (8.5 in x 11 in).
To make the khao dome, lay out a single banana leaf (sideways, or wider than it is tall). Plop a small dollop of the rice mixture in the center, then lay a slice of plantain on it, and add another dollop of rice on top of the plantain. Wet your fingers and use your hands to form the rice and plantain into a rectangle (taller than wide, opposite of the banana leaf). Now fold one side of the leaf over the rice, then fold the other side of the leaf back over the rice. Continue to wrap the leaf around the rice until you run out of leaf. Fold the top of the leaf down and the bottom of the leaf up; now you should have a nice little leaf package. You could tie it closed with some string, but I don't think it's necessary.
You'll need a steaming basket for this step. You can easily pick one up at a local asian market for a few dollars; I think it's a great investment! Stack the khao dome in the steam basket and cover. Fit the basket over a large pot with 2-3 inches of water in it, and heat the water to boiling over high heat. Steam the khao dome for 40 minutes and then remove them from the basket to cool.
Enjoy! These are great for on-the-go snacking, just peel the banana leaf back and use it to hold the rice.