Preheat oven to 325°F (180°C), in a bowl
Use a fork to mash the bananas. Add the stevia or sugar
½ cup (115 g) of melted butter, and vanilla, and whisk until combined. (If using stevia add vanilla yogurt at this step before combining)
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 20-25 minutes, until center no longer jiggles when pan is shaken
Allow brownies to cool completely before cutting