Preheat the oven to 350°F (180°C). Line three large baking sheets with parchment paper or silicone baking mats and set aside.
In a large mixing bowl, whisk together the flour, baking soda, ground cinnamon, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a handheld mixer, beat the butter, granulated sugar, and brown sugar together for 1 to 2 minutes or until well combined.
Mix in the mashed banana, egg yolk, and vanilla extract until fully combined, stopping to scrape down the sides of the bowl as needed.
Add the dry ingredients and mix until just combined, then mix in the chopped walnuts on low-speed until fully incorporated.
Using a 1.5 tablespoon cookie scoop, scoop the cookie dough and drop onto the prepared baking sheets, making sure to leave a little room between each ball of cookie dough.
Bake for 10 to 12 minutes or until the edges of the cookies are lightly browned and the tops are set. Remove from the oven and cool on the baking sheets for 5 to 10 minutes, then transfer the cookies to a wire rack to cool completely.
Store cookies in an airtight container at room temperature for up to five days.