Preheat the oven to 350°F (180°C).
Line two large cookie sheets with parchment paper. Slightly oil the paper with cooking oil spray. Set aside.
Mash the bananas and pack them in a 1-cup measuring cup to measure precisely.
In a large mixing bowl, combine all the ingredients: mashed banana, peanut butter, cocoa, oats, chia seeds, milk, chocolate chips, and any optional ingredient you choose to add.
Stir with a rubber spatula until fully combined, and a sticky chocolate cookie dough forms. If too dry, add a little more milk to bring together.
Use a cookie dough scoop to form 9 large cookie dough balls. Place each ball onto the prepared baking tray, leaving one inch of space between each of them. They won't extend in the oven; the shape you give now is the shape you get after baking.
The dough is sticky, and that's normal. Lightly oil the back of a fork and press down each cookie dough balls slightly to form a round cookie shape. if the sides cracks as you press wet your fingers and pat to form a nicely round cookie.
Bake the cookies for 20-24 minutes at 350°F (180°C) until dry on top.
Let the cookies cool on the cookie sheet for 15 minutes. They firm up as they cool down. Don't touch them. Sprinkle a few more chocolate chips on top, and let them melt for a boost of sweetness.
Transfer them to a cooling rack for 30 minutes.
