Preheat the oven. Set the oven to 347°F (175°C) and allow it to heat fully while the cookie mixture is prepared.
Prepare the banana mixture. Mash the ripe bananas in a mixing bowl until they have a smooth, even consistency.
Add the butter. Place the softened unsalted butter into the mashed banana and mix thoroughly until the two ingredients are well incorporated.
Form the dough. Gradually work the flour and baking powder into the banana-butter mixture. Add the scraped vanilla bean seeds, or the vanilla extract if using, and knead everything together until a smooth, uniform dough develops.
Chill the dough. Enclose the prepared dough in plastic wrap and refrigerate it for approximately 1 hour. Chilling firms the mixture and makes it easier to roll and shape.
Roll and shape. Lightly flour a clean work surface as well as the surface of the chilled dough. Roll it out to the preferred thickness, keeping in mind that the cookies will approximately double in thickness during baking.
Cut the cookies. Press Christmas-themed or other desired cookie cutters into the rolled dough and carefully transfer the shapes to a baking tray, leaving a little space between each cookie.
Bake. Place the tray in the preheated oven and bake for approximately 10 minutes, or until the edges of the cookies develop a light golden-brown color.
Cool completely. Allow the cookies to rest briefly on the baking tray before moving them to a wire rack to finish cooling.