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 mug with 6 to 8 banana leaf strips. Make sure the strips overlap each other. Turn the strips over the edge of the mugs. Place a rubberband over the strips around each mug to hold them down. Then tie kitchen twine over the leaves around the mugs. Remove the rubber band. Trim the twine and banana leaves so that they are neat.
Cut four round pieces of banana leaves the size of the base of the mugs. Place one piece in each mug. Set aside.
Pour water in a small saucepan. Turn on the stove to medium. Drop in pandan leaves and gula melaka. Cover saucepan and bring water to a boil. Remove the lid and stir to dissolve the gula melaka. Continue boiling for 3 minutes. Turn off stove and allow syrup to cool.
Sift glutinous rice flour into a large bowl. Remove pandan leaves from gula melaka syrup and pour into the glutinous rice flour. Stir to form a smooth batter.
Place the prepared mugs on a rack suitable for your slow cooker. Divide the batter into the 3 prepared mugs.
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 tikoy should turn a darker color when done. Remove from slow cooker after finished steaming. Allow the very soft and gooey tikoy to remain in the mugs at room temperature for 24 hours.
The tikoy should be firmer and the color darker after 24 hours. Remove them from the mugs. 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.