In a large bowl, combine the melted butter, brown sugar, and mashed banana. Mix to combine.

Add the egg yolk and vanilla extract. Mix well.

Add the flour, salt, baking soda, and cinnamon. Stir until combined and no streaks of flour remain.

Last, fold in the chocolate chips. Cover and set aside to let the flour hydrate in the dough while the oven preheats, about 15 minutes.

Line a cookie sheet with parchment paper and preheat the oven to 350°F. Once the oven is preheated, use a cookie scoop or a spoon to scoop out about 1 inch dough balls, placing them about 2 inches apart on the prepared cookie sheet. The dough will be a little wet! Bake for 10 to 11 minutes or until the edges are set and the cookies have puffed up, but the middles are still underbaked.

Allow to cool before eating!