Preheat oven to 350°F. Line two baking sheets with parchment paper.
In a medium mixing bowl, beat together the mashed banana, brown sugar, white sugar, melted butter, and vanilla extract until well combined.
Add the gluten free flour, cornstarch, baking soda, cinnamon, and salt to the bowl.
Beat until a soft dough forms and no dry streaks remain. Do not overmix.
Fold in the chocolate chips and chopped walnuts (if using) with a spatula.
Use a medium cookie scoop (about 1½ tablespoons) to portion the dough into balls and place on the prepared baking sheets, spacing a few inches apart. Place only 6 cookies per sheet.
Bake for 11–14 minutes, or until the edges are golden and the centers look just slightly underdone. They will continue to firm up as they cool.
As soon as the cookies come out of the oven, use a round cookie cutter slightly larger than each cookie and swirl it around each one to create perfectly round edges.
Let cool on the baking sheet for 15 minutes, then transfer to a wire rack.
While still warm, sprinkle with flaky sea salt and press extra walnut pieces and chocolate chips on top if desired.
Enjoy!