Preheat your oven to 350 degrees F. Line a baking sheet with parchment paper or a silicone mat. The banana makes these cookies sticky, so do not skip the lining.

In a large bowl, mash the banana until mostly smooth with just a few small lumps. Add the peanut butter, softened butter, egg, and vanilla. Beat with an electric mixer on medium speed until well combined, about 1 minute. The mixture will look slightly curdled at first but comes together.

Add the brown sugar and granulated sugar to the bowl. Beat on medium speed until creamy and the sugars are fully incorporated, about 30 seconds. Scrape down the sides of the bowl.

Add the flour, baking soda, cinnamon if using, and salt. Mix on low speed just until no dry flour remains. Do not overmix - the dough will be soft and slightly sticky, more like thick cake batter than traditional cookie dough.

Place the 2 tablespoons granulated sugar in a small shallow bowl. Using a cookie scoop or two spoons, portion dough into 1.5 tablespoon balls. Roll each ball gently in the sugar to coat completely. This creates a slight crackly crust and prevents sticking.

Arrange dough balls 2 inches apart on the prepared baking sheet. Bake for 11-13 minutes until the edges are set and the tops look dry with a few cracks. The centers will still look slightly underdone - this is correct. They will look puffy and pale golden, not deeply browned.

Let cookies cool on the baking sheet for 5 minutes - they need this time to set up. Then transfer to a wire rack to cool completely. The cookies will deflate slightly and develop their characteristic chewy texture as they cool.