Preheat your oven to 350°F (175°C). Grease the baking pan or line it with parchment paper for easy removal.
In a mixing bowl, mash the ripe bananas until smooth. You should have about 1 ½ cups of mashed bananas.
Add the vegetable oil and brown sugar to the mashed bananas. Mix well until combined.
Crack the eggs into the mixture; add the vanilla extract and whisk until everything is fully incorporated.
In another bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Do not overmix.
If desired, fold in the walnuts or chocolate chips for added texture and flavor.
Pour the brownie batter into the prepared baking pan and spread it evenly.
Bake in the preheated oven for about 30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the brownies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.