Preheat the oven to 350°F (180°C). Line an 8-inch x 8-inch brownie pan with parchment paper. Set aside.
In a high-speed blender, add ripe banana pieces, coconut oil, peanut butter, vanilla extract, coconut sugar, unsweetened cocoa powder, salt, baking powder, and rolled oats. If you want to use all-purpose flour, don't add in the blender, or the brownie will be ultra gummy. Set aside in a bowl and blend the remaining ingredients
Blend on high speed until smooth and well combined with no lumps. You may have to stop the blender a few times, scrape down the side of the jug, and repeat, or use the tamper attachment tool to push ingredients next to the blade.
If you used all-purpose flour, transfer the batter onto the bowl with flour, stir to combine, and form a thick batter.
Transfer the brownie batter onto the prepared pan.
Use a spoon to add dollops of peanut butter on top of the batter.
Use a skewer to swirl the peanut butter into the brownie batter and create a swirl effect.
Bake for 20-25 minutes or until the batter is set - if you move the pan the batter won't jiggle or move.
Let the brownies cool down for 10 minutes in the pan, then pull out the hanging pieces of parchment paper to transfer the brownie onto a cooling rack.
Cool completely - about 1 hour at room temperature - before cutting into 16 squares.
Store brownies in an airtight box in the fridge for up to 5 days or freeze.