Line a small baking dish (like an 8x8 inch) with parchment paper or grease it lightly.
In a large bowl, mash the bananas with a fork or potato masher until smooth.
Add the peanut or almond butter and cocoa powder to the mashed bananas. Stir vigorously with a spatula until well combined and there are no dry streaks.
Optional Add-Ins (mix these in before pouring batter into the pan):
You can add nuts, chocolate chips, or a pinch of salt to enhance the flavor.
Baking:
Pour the batter into the prepared dish and spread it out evenly.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out mostly clean.
Let the brownies cool completely in the pan before cutting and serving. This allows them to set properly.
Tips:
The riper the bananas, the sweeter and more flavorful your brownies will be.
Store leftover brownies in an airtight container at room temperature for up to 3 days.