Preheat the oven to 350 degrees F (177 degrees C).
First, brown ½ cup of unsalted butter. Melt the butter over medium heat until fully melted. Lower to medium low and cook until brown bits start to appear. Remove from the heat and set aside.
Mix together 1 ¼ cup of all purpose flour, ½ tsp of baking soda, ½ tsp of ground cinnamon, and ¼ tsp of salt. Set aside.
Combine browned butter and 1 cup of light brown sugar in a bowl. Whisk until combined.
Add the egg yolk and whisk until combined.
Add ½ tsp of vanilla extract, 1 small mashed banana, and 2 tbsp of white miso. Whisk until light and fluffy (see image for reference). Make sure to use only 1/4 cup of mashed banana. Otherwise, the cookie will become cakey.
Combine the wet ingredients with the dry ingredients from step 3. Mix until just combined.
Finally, add ¼ cup of roughly chopped pistachios and 2 oz of dark chocolate. Mix until just combined.
Prep 2 baking sheets by covering them with parchment paper. Using a medium cookie scoop (#16 cookie scoop), scoop the dough onto the prepped baking sheets.
Bake for 12-14 minutes until the edges are slightly brown.
Cool for 5-10 minutes before transferring to a cooling rack to cool completely.