2 cups (10 ounces) all-purpose flour (see note about using whole wheat)
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1/4 cup (.75 ounces) natural, unsweetened cocoa powder
1/2 to 3/4 cup chocolate chips
Preheat the oven to 350 degrees F. Lightly grease a 9X13-inch baking pan and set aside. I use a metal baking pan for this recipe; a glass baking pan may require additional baking time.
In a large bowl, whisk together the butter, sugar, egg, vanilla, buttermilk (or sour cream), and mashed bananas. Add the flour, baking powder, baking soda and salt and mix until no dry streaks remain (don't overmix).
Scoop half of the batter into a second bowl (about 20-22 ounces of batter if you are weighing your ingredients). Add the cocoa powder to this bowl and mix until evenly combined.
Spread the chocolate batter in the prepared pan. It will be a thin layer but should cover the bottom of the pan.
Dollop the banana bread batter in large spoonfuls over the top and spread in an even layer across the chocolate batter.
Sprinkle the chocolate chips across the top. Bake for 30-35 minutes until the bars are puffed and a toothpick inserted in the center comes out with moist crumbs (but not wet batter). The top should spring back lightly to the touch when pressed.
I think these bars would work quite well with whole wheat flour in place of some of the all-purpose flour. I'd probably start with half all-purpose, half whole wheat (I always suggest using white whole wheat flour for a lighter texture).
Ripe bananas are key for good texture and perfect sweetness! Any green on the banana? It's a no-go. Look for brown spots.