Break 200g plain chocolate (I used 70% cocoa) into cubes and put into a pan with 600ml full-fat milk. Gently heat until the chocolate melts, then bring almost to the boil, whisking regularly until even and smooth. Can be made ahead, then chilled for up to 2 days. Re-heat if necessary, whisking, then ladle or pour into heatproof glasses or mugs. Top with scoops vanilla ice cream and mini marshmallows. Add a nip of rum or brandy for the adults, if you like.