Tortillas will keep in the refrigerator, in a sealed plastic bag, for up to two months.
Preheat oven to 450 degrees. Place tortillas on a baking sheet; brush both sides of tortilla with melted butter. Bake until golden, 7 to 8 minutes (if they puff up, press down with a spatula).
In a small saucepan, stir brown sugar, cinnamon, and 1 tablespoon water over medium heat. Stir in 4 tablespoons butter until melted.
Slice bananas diagonally 1/4 inch thick; arrange in overlapping circles on tortillas. Brush brown-sugar mixture evenly over galettes. Bake until shiny, about 10 minutes. Cut into wedges; serve warm, with ice cream, if desired.