Preheat your oven to 350 degrees
In a medium-sized bowl, mash the bananas together using a fork
Add in the sugar, ½ cup melted butter, and the vanilla and whisk until combined
Add in the flour, salt, and cocoa powder and continue to whisk until fully combined
Using the remaining tbsp of butter, grease a 9x9 square baking dish
Sprinkle the remaining 1 tbsp of cocoa powder on every side of the greased baking dish
Pour the brownie batter into the prepared baking dish
Spoon the peanut butter over the brownie batter, distributing it evenly
Use a butter knife to draw swirls of peanut butter into the batter
Bake in preheated oven for 25-30 minutes until the center is set