Preheat oven to 350°F. Line with parchment or grease an 8x8 inch baking pan.

Combine dry ingredients: Stir the ¾ cup all-purpose flour, ½ teaspoon baking powder, and ½ teaspoon salt together in a bowl.

Brown butter: In a medium sauce pot over medium heat, melt ½ cup unsalted butter. Continue to cook while it foams and whisk constantly until it browns and you see some brown specks at the bottom and it starts to smell a bit toasted. Remove from heat.

Mix base ingredients: Once the butter cools a bit, add 1 ½ cups granulated sugar and 2 teaspoons pure vanilla extract. Next, beat in 3 eggs one at a time with a spoon. Gradually add the dry ingredients and stir the batter until it's evenly moistened. Divide the batter in half into two separate bowls.

Mix the brownie portion: Add the ½ cup cocoa powder and ½ cup mini chocolate chips to one of the bowls and stir until combined. This mixture will be very thick.

Mix the banana bread portion: In the second bowl of batter, stir in the mashed 1 cup ripe bananas.

Prepare banana bread brownies: Spread ½ of the chocolate batter into the bottom of the prepared baking pan, and follow with all of the banana batter. For the remaining chocolate batter, you can either spoon it on top or you can put it in a plastic bag, snip off the corner, and squeeze it onto the banana batter in lines. Use a butter knife or a spatula to swirl the two batters together to create a marbled appearance.

Bake: Place the baking dish in the preheated oven and bake until the brownies begin to pull away from the sides of the pan, and a toothpick inserted into the center comes out clean, about 40 minutes. Cool them in the pan, cut into squares, and serve.