Turn a burner on to low heat. Using tongs, slowly pass each banana-leaf piece over the flame on both sides and set aside (so they will be resilient, malleable and not break).
One by one, set each leaf piece on your counter with the shiny, outer side down. Spoon about 1/4 cup of masa into the center and spread to form a rectangle of about 4”x 3”. With the spoon, make a shallow channel down the middle, creating a stripe in the masa. Spoon a couple tablespoons of the seasoned shredded chicken right down the middle. Gently close each tamal by folding the longer sides first and then the sides as if making a flat and tight package, but being careful not to press on the tamal too much.
Prepare your tamalera or steamer: Add just enough water to touch the bottom of steaming basket and a coin (it will jump and make noise to let you know if water runs out). Line the steaming basket with a few banana-leaf pieces to gently cover the base. One by one, add the tamales, stacking them as evenly as you can, staggered in the same position as when you made them: laying them flat, with opening side on the top. Once you are done, cover with a few more pieces of banana leaves.
Set the steamer uncovered over high heat, once there is a bit of steam coming out and the water starts boiling a few minutes later, cover, reduce heat to medium, and cook for 50 minutes to an hour. The leaves will have changed to a much darker color and will have completely wilted to wrap themselves as a second skin over the tamales, and the tamales should feel firm. Turn off the heat.
Let the tamales sit covered for 10 to 15 minutes - so they firm up - or until ready to serve. The tamales can be made ahead of time, and reheated in steamer. They can be refrigerated up to 5 days, or frozen in sealed plastic bags for 6 months, and reheated the same way.