Put the double cream and milk in a bowl and whisk until just stiff. Stir in the icing sugar and vanilla essence. Pour the mixture into a shallow freezer container and freeze for 30 minutes or until the ice cream begins to set around the edges. (This ice cream cannot be made in an ice-cream machine.)
Melt the chocolate (see
Remove the ice cream from the freezer and spoon into a bowl. Add the melted chocolate and quickly stir it through the ice cream with a fork. Return the ice cream to the freezer container, cover and freeze until set. Transfer the ice cream to the refrigerator 30 minutes before serving, to soften slightly.
Heat all the ingredients for the chocolate sauce, if making, gently in a saucepan, stirring until melted. Serve immediately with scoops of the ice cream.