Prepare the Bananas: Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving some overhang for easy removal. Mash the ripe bananas in a bowl until mostly smooth.
Mix the Wet Ingredients: In a large bowl, whisk together the melted butter, granulated sugar, brown sugar, mashed bananas, eggs, and vanilla extract until smooth and well combined.
Add the Dry Ingredients: Add the all-purpose flour, cocoa powder, and salt to the wet mixture. Gently fold with a spatula just until the dry ingredients are incorporated. Avoid overmixing.
Fold in the Chocolate Chips: Gently fold the semi-sweet chocolate chips into the brownie batter. Reserve a few chocolate chips for sprinkling over the top if desired.
Transfer to the Pan: Pour the batter into the prepared baking pan and spread it evenly into the corners. Sprinkle additional chocolate chips and chopped walnuts over the top if using.
Bake the Brownies: Bake at 350°F for about 23–27 minutes, or until the edges are set and the center still looks slightly soft. Do not overbake if you want a fudgy texture.
Cool Completely: Remove the pan from the oven and place it on a cooling rack. Let the brownies cool completely in the pan before lifting them out with the parchment paper.
Slice and Serve: Cut the cooled brownies into 16 squares. Serve as they are or top with ice cream, whipped cream, chocolate sauce, or fresh banana slices.