Scrub banana leaves with a washcloth under running water. Place banana leaves in a pot submerged in water and bring it a boil on the stove. Turn off stove and allow banana leaves to soften in boiling water for 10 minutes.
Remove banana leaves from hot water and wipe them dry. Cut the banana leaves into 2½ inch wide strips.
Line each ramekin with 4 to 6 banana leaf strips. Make sure the strips overlap each other. Turn the strips over the edge of the ramekins. Place a rubberband over the strips around each ramekin to hold them down. Then tie kitchen twine over the leaves around the ramekins. Trim the twine and banana leaves so that they are neat. Remove the rubber band.
Cut four round pieces of banana leaves the size of the base of the ramekins. Place one piece in each ramekin. Set aside.
Combine ½ cup (120ml) water, pandan leaves, and coconut palm sugar in a small saucepan. Turn on the stove to medium. Stir to dissolve the coconut palm sugar. Then bring it up to a boil and let it continue boiling for 3 minutes. Turn off stove and allow syrup to cool.
Sift glutinous rice flour into a large bowl. Pour in remaining ½ cup (120ml) water. Mix with a spatula.
Remove pandan leaves from coconut palm sugar syrup and pour into the glutinous rice flour mixture. Stir to form a smooth batter.
Divide the batter into the 4 prepared ramekins and place the ramekins on a rack suitable for your slow cooker.
Transfer the rack to your slow cooker insert filled with 1 to 2 inches of hot water. Place a clean kitchen towel over the top and place the lid on top of the kitchen towel.
Set the slow cooker to cook on high for 8 hours. Check and top up water in the slow cooker insert every two hours.
The nian gao batter should turn a golden color when done. Remove from slow cooker after finished steaming. Allow the very soft and gooey nian gao to remain in the ramekins at room temperature for 24 hours.
The nian gao should be firmer and the color darker after 24 hours. Remove them from the ramekins. Trim the banana leaves and allow them to become firmer for 3 to 4 more days at room temperature before serving.
Store in the refrigerator for up to 6 months.