Preheat oven to 350 degrees F. Line baking sheets with parchment paper. Set aside.
In the bowl of your stand mixer fitted with the paddle attachment or with an electric hand mixer in a large bowl, beat together butter and sugars until well combined.
½ cup unsalted butter, ½ cup light brown sugar, ⅓ cup granulated sugar
Add the banana, egg yolk, and vanilla. Mix until combined.
½ cup mashed banana, 1 large egg yolk, ½ teaspoon vanilla extract
Add flour, baking soda, and salt. Stir in until just combined.
1 ½ cups all-purpose flour, ½ teaspoon baking soda, ¼ teaspoon kosher salt
Stir in oats and chocolate chips.
1 cup old fashioned oats, ⅔ cup semi sweet chocolate chips
Use a 1 ½ tablespoon cookie scoop to scoop cookie dough and place on prepared baking sheets, about 2 inches apart.
Bake for about 10-15 minutes, or until cookies are set around the edges but still soft in the center.
Let cookies cool completely before removing from the baking sheets.
Notes
Butter: You can use salted butter in place of the unsalted butter and omit the salt.
Banana: It's important to use overripe bananas in this recipe for maximum flavor and moisture. The best bananas to use will be yellow with brown spots.
Egg yolk: Egg yolk helps to add richness and chewiness to the cookies.
Flour: To properly measure the flour, either weigh it (click "metric" just above the ingredients for the weights) or sift the flour, lightly spoon it into a measuring cup, and level without packing the flour.
Chocolate chips: You can use semi-sweet chocolate chips, milk chocolate chips, or dark chocolate chips in this recipe. You can also use a combination of chocolate chip and peanut butter chips.