In a small bowl, stir together the flax meal and water. Allow this to rest for 10 minutes, occasionally stirring to ensure it thickens. This is called a flax egg.
Add the all-purpose flour, baking soda, cornstarch, pumpkin pie spice, and salt to a bowl, and mix well. Set aside.
To a standing mixer, add the butter and brown sugar. Using the paddle attachment, beat this at medium-high speed for 2 minutes, until slightly light and fluffy. Scrape down the sides of the bowl as needed. If you don't have a standing mixer, you can do this in a bowl with a wooden spoon.
Add the flax egg and vanilla extract to the mixing bowl and beat for another 2 minutes.
Turn the mixer down to the lowest speed and add the mashed banana. Then, slowly dump the dry ingredients into the bowl. Beat the ingredients until just combined. Do not over-mix.
Now, add the chocolate chunks and walnuts into the mixing bowl and stir with a rubber spatula or a wooden spoon until evenly combined.
Line a large sheet pan with a piece of parchment paper and scoop 2 tbsp to 1/4 cup portions(depending on how big you want the cookies) of cookie dough onto the pan, leaving at least 2 inches apart from each other. Do not overcrowd the pan with cookie dough, as they will spread in the oven.
Press a few chocolate chunks on top of each cookie dough ball.
Place the pan in the refrigerator while you preheat the oven to 350 degrees Fahrenheit. Place a piece of plastic wrap onto the remaining cookie dough still in the mixing bowl, and let it chill in the fridge as well.
Once the oven is preheated, bake the cookies on the middle oven rack for 9-12 minutes. For smaller cookies, bake them for around 9 minutes or until your desired doneness. For 1/4 cup-sized cookies, I typically bake them for 12 minutes!
Allow the cookies to cool on the sheet pan before moving them. They are somewhat fragile when hot.
Repeat the last four steps of the recipe until you have no cookie dough left!