Preheat oven to 350F/180C. Line a 9x13-inch baking pan with aluminum foil, grease it lightly with non-stick cooking spray, and set aside.

In a large microwave-safe bowl add butter and sugar and warm it up in small increments in the microwave oven until the butter and sugar melt and stir it well using a whisk.

To this mixture add the plain yogurt, mashed banana, and vanilla extract and mix well.

Stir in the all-purpose flour, cocoa powder, baking soda, and salt and combine it until smooth using a whisk.

Incorporate the chocolate chips and walnuts, if using.

Pour the batter into the prepared pan and tap it well enough to break any air bubbles.

Bake it for 35-40 minutes or until a toothpick inserted in the middle comes out having just little crumbs. Mine was ready after 37 minutes.

Remove it from the oven and place the pan on a wire rack for about 15 minutes and then using the aluminum foil lift the brownies off the pan and let it cool on the wire rack at least for a couple of hours before you can cut it into squares. I left mine to cool overnight because I baked the banana brownies at night.

Cut it into squares and store it in an air-tight container in the refrigerator because it's a moist brownie and has bananas so not recommended to leave it outside unless you plan to finish it off within a day or two. These brownies will freeze well. Just thaw it for a couple of seconds in the microwave oven before consuming it.