Preheat oven to 350°F (180°C). In a bowl, use a fork to mash the bananas. Add the sugar, ½ cup (115 g) of butter, and vanilla, and whisk until combined.
Add the flour, salt, and cocoa powder, and whisk until fully incorporated. Use the remaining 1 tablespoon of butter to grease a 9x9-inch (23x23 cm) baking tray. Use the remaining 1 tablespoon of cocoa powder to dust every side of the baking tray.
Pour batter into prepared baking dish. Use a spoon to add dollops of peanut butter on top of the batter. Use a butter knife to swirl the peanut butter into the brownie batter. Bake for 25-30 minutes, until center no longer jiggles when pan is shaken. Allow brownies to cool completely before cutting. Enjoy!