Preheat oven to 180 degree c.
Line a square pan with parchment paper.
Beat butter, sugar, vanilla till creamy.
Add banana and beat till mixed.
Add eggs and beat till fluffy.
Add in flour, cocoa, baking soda and fold gently.
Add in chocolate chips and fold gently
Spoon this into baking pan and dollop peanut butter all over the cake. Swirl it.
Bake for 30 mins
Remove and cool completely.
Slice and serve.