Wipe the banana leaves clean, then pass them quickly over a low flame to soften. Remove the thick center rib and cut into pieces, saving a few strips for tying.
Rinse the glutinous rice until the water is mostly clear. Drain well.
Combine the coconut milk, water, sugar, and salt in a wide pan. Stir and bring to a boil.
Add the rice, lower the heat, and cover. Cook for about 15 minutes, stirring occasionally, until most of the liquid is absorbed. Remove from the heat and cool slightly.
Place about ¼ cup of the rice mixture on a banana leaf and shape it into a short log. Fold the leaf over the rice, roll loosely, and fold both ends underneath. Tie two pieces together with a strip of banana leaf.
Place the wrapped suman in a pot and cover with water. Bring to a boil, then lower the heat and cook gently for about 30 minutes, until the rice is tender. Keep the suman under the water and add more boiling water as needed.
Let cool slightly before unwrapping. Serve with sugar or latik.