1. Preheat your oven to 350°F (175°C) and grease a 9×9-inch baking pan or line it with parchment paper.
2. In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. This step ensures that all the dry ingredients are well combined and aerated.
3. In a large mixing bowl, combine the melted butter and sugar, beating until smooth and creamy. This should take about 2 minutes.
4. Add the eggs and vanilla extract to the butter mixture, and mix until fully incorporated.
5. Gently fold in the mashed bananas, making sure not to overmix. The batter should be slightly lumpy.
6. Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix, as this can make the bars dense.
7. Fold in the chocolate chips and walnuts, if using, for added texture and flavor.
8. Pour the batter into the prepared baking pan, spreading it evenly with a spatula.
9. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to keep the bars fudgy.
10. Let the bars cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely. Slice into squares and enjoy!

Remember, cooking is an art, not a science. Feel free to adjust the sweetness or flavors to suit your preference. As my abuela always said, “Sigue tu corazón en la cocina” — follow your heart in the kitchen.