Finely chop or grate the chocolate and place in a saucepan with the sugar and milk and bring to the boil until the chocolate is completely melted. Then add the cocoa. Let the mixture cool down and put it in a blender along with the peeled banana. Mix in the banana and puree. Let the mixture cool completely. Pour into an ice cream maker and let it set to a creamy ice cream in approx. 20 - 30 minutes.