Preheat the oven to 350°F and line two baking sheets with parchment paper.
In a large mixing bowl, mash the banana with a fork. Add the butter, brown sugar, egg yolk, and vanilla extract. Using a handheld mixer, mix on medium speed for 30 seconds, or until just combined.
Add the flour, baking soda, and salt. Mix until no flour remains, scraping down the bowl as needed.
Fold in the peanut butter chips.
Let the cookie dough rest at room temperature for 10 minutes. (Don't skip this step!)
After the dough has rested, use a 1 tablespoon cookie scoop* to drop the dough onto the lined baking sheets, spacing each about 2 inches apart.
Bake for 7-8 minutes, or until the cookie edges are set (the middle will still appear underbaked).
Transfer the cookies to a cooling rack to cool for 5-10 minutes before enjoying.