Preheat the oven to 180 degrees/ 350 F. Grease a 20cm square baking tray with a little oil, and line with greaseproof paper.
Break up the 4oz/ 115 grams chocolate and melt until smooth (I like to do this in the microwave, by carefully heating on the medium setting in 30 second bursts and stirring after each interval, but you can use a double boiler if you wish). Set aside to cool.
In a large mixing bowl, mash the banana until no large lumps remain (you can puree the banana in a blender if you wish). Add the sunflower oil and sugar, and whisk until well combined.
Pour in the cooled melted chocolate and whisk together to combine. Stir in the vanilla extract and espresso powder, if using.
Measure out the dry ingredients into a separate bowl. Gradually sift them in, folding the batter after each addition (I would do this in 3-4 increments).
Add the chopped chocolate to the batter and mix to combine. Pour into the baking tray and swirl through the peanut butter.
Bake for 20-25 minutes, then remove from the oven and leave to cool completely before cutting into squares. Enjoy!