In order to prevent water coming in the cake during cooking process, I recommend you to wrap each log with plastic wrap then double wrap with aluminum foil on the outside before boiling them in water.
Use a big soup pot, put the cake vertically (preferred) or horizontally depends on the size of your pot. Fill the pot with water, make sure all the banana cakes submerged completely under water (ideally at least 2 inches).
Bring to boil then cover the pot, adjust the heat so the water still hard boiling but not spilling all over the place.
Check the pot and fill with hot-boiling water if the water level drop below the cakes every 1.5 hours.
The banana cakes (bánh tét chuối) should be done after 4 hours or max 5 hours, you’ll be able to smell the banana and coconut milk at this point.
Discard the plastic wrap and aluminum foil. Let the cakes cool down completely to room temperature before serving, approximately 2 hours.