Preheat the oven to 170 / 150 fan or Gas 3
Line a 2lb loaf tin with baking paper or liner
Beat together the butter and sugar until fluffy
Add one egg and mix until combined
Add half the flour and mix until combined
Repeat with the remaining egg and flour
Add vanilla essence, mashed banana and chopped chocolate and fold into the mixture just until combined (do not over mix)
Put mixture in the tin and sprinkle dark brown sugar over the top
Cover loosely with foil and place in the oven (middle shelf )
Bake for 50 mins, remove the foil and then continue to bake
Bake for the remaining 10 minutes or until cake is golden brown and is springy to the touch
Leave in the tin to cool completely
Devour with a steaming mug of hot chocolate to wash it down!