Preheat your oven to 180C /350F / gas 4 and line a 23 X 23 cm (9 inch by 9 inch) cake tin / brownie pan with grease proof paper.
Mash the bananas in a large bowl using a fork.
Next add in the sugar, oil, milk and just 1 tbsp of the peanut butter and mix until combined.
Then mix in the flour, cocoa powder and chocolate chips.
Pour the brownie batter into the lined cake tin / brownie pan.
Heat the remaining peanut butter in the microwave until it's a runny consistency (about 30-45 seconds) and drizzle over the top of the batter. Swirl using a skewer.
Bake for approximately 30 minutes, until the brownies start shrinking away from the sides of the tin, the centre should feel a bit springy and a skewer inserted should comes out cleanly.
Allow the brownies to cool completely before cutting and serving.