Heat the milk with the chocolate in a saucepan and let the chocolate melt in it. I always whisk everything together with a small milk frother. Peel the banana, cut into pieces and add. Puree everything finely with a hand blender.
But it also works the other way around. First make the banana milk with the hand blender, put everything in a saucepan, add the chocolate and heat everything until the chocolate has melted. Possibly whisk with the milk frother.