Preheat oven to 180 degree C. Line a 20 cm square pan with parchment paper and set aside.
Take banana, sugar, milk, water, oil, vanilla in a bowl and mix well.
Sieve over flour, cocoa powder, baking soda, baking powder, salt and mix well.
Spoon into the pan and bake for 30 to 35 mins.
Remove and cool completely.
Make icing by melting chocolate and butter in a sauce pan.
Pour this over the cake and let it set in fridge.
Slice and serve.