Pour the milk, cocoa powder and sugar into your slow cooker pot. Whisk well.
Heat on high for 2 hours.
Add the chopped chocolate and stir until melted.
Make a smooth paste using 8 teaspoons (4 dessert spoons) of milk and 4 teaspoons (2 dessert spoons) of cornflour. Pour the cornflour mixture into the hot chocolate and whisk well.
Heat on high for 1 hour until thickened. Whisk occasionally to prevent lumps.
Transfer to a jug and serve. Top with whipped cream and chocolate shavings.