Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking pan or line it with parchment paper for easier removal.
In a large mixing bowl, combine the mashed bananas, melted coconut oil, brown sugar, and granulated sugar. Mix until well combined.
Add in the eggs and vanilla extract, whisking until the mixture is smooth.
In a separate bowl, sift together the cocoa powder, flour, baking soda, and salt to avoid lumps.
Gradually incorporate the dry ingredients into the wet mixture, stirring until just combined. Do not overmix.
If using, fold in the chocolate chips and chopped walnuts until evenly distributed.
Pour the brownie batter into the prepared baking pan, spreading it evenly.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.
Notes
Cut the brownies into squares and dust with powdered sugar for a beautiful finish. Serve warm with a dollop of whipped cream or a scoop of vanilla ice cream for an indulgent treat!