Preheat the oven to gas mark 4/350oF/180oC and line a rectangle baking tin which measures approximately 13 x 9 inches.
In a bowl, combine all the dry ingredients (plain flour, salt, cornflour, baking powder and bicarbonate soda. Set aside.
In a microwavable bowl, melt the butter and margarine using 40 second increments. You can also do this in a saucepan on the cooker. Set the melted butter aside to cool.
Using a fork, mash the banana and set aside. Then whisk the eggs in a small bowl and set aside.
The butter should be cooled now so add the brown sugar and granulated sugar then combine using a rubber spatula. Add the vanilla extract and the whisked eggs then combine with the butter and sugar. Then add the mashed banana, peanut butter and mix them in well. Add the chocolate chips and also mix in well.
Add half the flour to the mixture and combine well. Add the second half and also combine well. Pour the batter into the prepared baking tin and bake for 35 minutes. Once baked, allow to cool completely before removing from the tray and cutting.