Roughly chop 75g of the chocolate and put into a large jug with the coffee. Bring 300ml of the milk just to the boil, then pour over the chocolate and coffee mix, stirring to melt. Once melted, cool.
Tip the cooled mocha milk into the blender with the ice cream and remaining milk. Blitz until blended, then pour into 4 tall glasses. Add an extra scoop of ice cream to each, if you like, then grate a little of the remaining chocolate on top.