Peel the bananas and place them in a large mixing bowl.
First mash the bananas lightly with a fork, then blend or mix them until you get a smooth banana puree with no large chunks.
Melt the butter, pour it over the banana puree, and mix until fully combined.
Add the unsweetened cocoa powder to the bowl and mix until the color is evenly chocolatey.
Sprinkle in the baking powder and mix again so it is evenly distributed throughout the batter.
Use a hand mixer or whisk to beat the mixture until the batter looks smooth and homogeneous, with no visible streaks.
Line a baking pan with parchment paper, pour in the batter, and spread it out in an even layer with a spatula.
If you like extra crunch, sprinkle the top with chopped peanuts before baking.
Bake in a preheated oven at 350°F (180°C) for about 18–20 minutes, until the edges are set but the center is still slightly soft.
Let the brownie cool completely, then, if desired, dust the top with powdered sugar before slicing and serving.