Preheat the oven to 350°F. Line a 9x9 square baking pan with parchment paper and coat with cooking spray. Set aide.
In the bowl of a stand mixer fitted with the paddle attachment or the bowl of a large mixing bowl with a handheld electric mixer, combine the bananas with sugar, melted butter, vanilla and egg.
Sift or whisk together the flour, cocoa powder, baking powder and salt.
Slowly add dry ingredients to wet ingredients until just combined.
By hand, fold in the peanut butter chips.
Pour the batter into the prepared baking dish.
Heat the peanut butter in a microwave-safe bowl or glass measuring cup for 30 seconds-1 minute just to loosen it. Dollop onto the brownies in 9 spoonfuls. Using a knife or toothpick, swirl through the batter as best you can.
Bake for 25-30 minutes or until the center is set and no longer jiggly. These are fudgy brownies, so the toothpick test isn't always reliable.
Remove and allow to cool fully before removing using the parchment paper like a sling. Cut into 9 large bars (or 18 smaller bars).
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Notes
Store this Peanut Butter Banana Brownies recipe in an airtight container. Just like regular brownies, they will be good for up to five days total.