Crème Chantilly is not just whipped cream; it must be flavoured with vanilla, be lightly sweetened and very cold. Whipping cream is not the variety to use as it breaks down too easily, so use double cream. To give lightness, crushed ice is added halfway through the whipping process. This may seem an odd thing to do but it helps (a) to keep the cream cold, and (b) the addition of a little water (melted ice) adds to the insubstantiality, which is the secret of a good crème Chantilly.