Combine the butter and walnuts in a medium saucepan. Heat to melt the butter and continue cooking as it bubbles, stirring frequently to prevent it from burning. Remove from heat when the walnuts smell toasty and the white bits in the butter turn brown.

Pour the browned butter and walnuts into a large heat-safe bowl. Add cinnamon and whisk to combine. Let it cool for a few minutes.

Add brown sugar and granulated sugar into the browned butter. Whisk to combine.

Add the egg yolk, vanilla extract, and salt into the same bowl. Break one medium banana into pieces and add it in (about ⅓ cup). Whisk to combine.

Add flour and baking soda into the wet ingredients. Stir until it forms a dough.

Add the chopped chocolate and break up the second medium banana into coarse pieces. Fold them into the cookie dough.

Refrigerate the cookie dough for 1 hour until it firms up.

Preheat the oven to 355°F (180°C). Line a baking sheet with parchment paper.

Use a two tablespoon scoop to scoop balls of cookie dough and bake them in batches, leaving a few inches between the cookies for them to spread. Bake the cookies for 10-12 minutes and let them cool on the cookie sheet for a few minutes to firm up.