Preheat the oven to 190 C / 170 C fan / Gas 5. Grease and line a 900g (2 lb) loaf tin with baking parchment.
In a large bowl mash the bananas with the back of a fork and stir through the cooled butter, eggs and sugar.
Mix together the dry ingredients with a pinch of salt, then sift this over the mashed bananas. Half fold through with a large metal spoon, then add the milk and chopped chocolate and continue to carefully fold everything together. Spoon the mixture into the prepared tin.
Bake in the preheated oven for 1 hour, or until a skewer inserted into the middle comes away clean (cover with foil if it browns too much). Leave to cool for 15 minutes in the tin, then turn out onto a wire rack to cool completely.