Pour the cornflour into a jug and mix in 1 tablespoon milk to make a smooth paste. Add 200 ml (7 fl oz) of the milk, the sugar, the chocolate and the alcohol.
Microwave the mixture on high (900 watts) for 2 minutes or heat in a pan on the hob. Pour the hot chocolate into a tall glass mug.
Heat the remaining milk and froth vigorously. Pour over the hot chocolate. Sprinkle over some grated chocolate and serve immediately.