Preheat oven to 350°F. Line a 8 x 4 inch loaf pan with parchment paper.
Mash bananas until they become a puree consistency and no banana chunks remain. You can mash by hand with a fork or spoon or use a blender or food processor. Whisk in the cocoa powder until a smooth batter forms.
Pour batter into prepared baking pan. Use spatula to spread the batter evenly across the pan. Tap the bottom of the pan a few times against your counter to remove any air bubbles.
Bake for about 20-25 minutes. When the brownies are done, the surface should look dry and dark in color and the center should not be jiggly. However, the center should also not be completely set (i.e., a toothpick inserted will come up wet and if you press your finger gently on the surface you should feel that the top is firm but the inside is still soft). Let brownies cool in pan. Then place into the fridge for 1-2 hours or until brownies are firm enough to cut.
To make the frosting, add oil and cocoa powder in a medium bowl. Whisk until smooth and completely combined. Your mixture should look like melted chocolate. Add in maple syrup. (Do not add maple syrup until the end. The maple syrup will thicken the mixture. If you add it with the other ingredients, it will make it very difficult to mix.) Whisk until smooth frosting forms. Spread frosting over the surface of brownies after they are set.
Notes
Do not skip out on refrigerating the brownies. If you do not refrigerate them, they will not set and the inside of the brownies will remain like a thick pudding. Once they are refrigerated, they will stay set and you can leave them out at room temperature for several hours.
Store uneaten brownies in the fridge.
This is meant to be a low sugar recipe. The brownies have some sweetness (similar to bittersweet or dark chocolate) but they are not super sweet. You can taste the batter before baking and if you want sweeter brownies you can add some maple syrup to the batter or chocolate chips. You can also frost them after.
The outside of the brownies will be very dark, almost black when they are done. This is normal.
You can use either dutch process or natural cocoa powder. I used dutch process. I recommend dutch process if you have it because it is less bitter than natural cocoa powder.
I don't recommend baking in a pan bigger than 8 x 4 inch because your brownies will be too thin. If you want to use an 8 x 8 pan, you will want to double the recipe.
Nutrition estimate is for the brownies only, without any frosting.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.