One should refrigerate a large glass with the required amount of sweetened coffee, and add the ice cream and chantilly just prior to serving. Often crushed roasted coffee beans are put on top of the chantilly as decoration.
Contrary to appearance the café liégeois was not created in Liège. In fact, it was originally known in France as a café viennois (Viennese coffee). However during World War I with the Battle of Liège in full swing and Vienna representing the enemy, Paris' cafés started renaming the dessert café liégeois in honour of Belgium's embattled forts. Curiously, for a while at least, in Liège itself it continued to be known as café viennois.