The classic cake base for the petit four is a sponge cake called génoise. Named for the city of Genoa (an important city in the history of French cuisine — even thought it’s in Italy), génoise, due to its light texture and pliability, is the base cake for a wide range of pastries — especially the rolled-up variety like ice cream rolls, jelly rolls, and Christmas bûches de Noël. It’s also great for horizontal slicing, which is what petit four assembly requires.
Of course the pastry police aren’t going to hunt you down and clap you in irons should you decide to use something else (their methods are far more, how shall I say…subtle). Though génoise is a rather light cake (usually), more than a few bakeries use butter cake, which, being much more crumbly and difficult to slice (especially into thin sheets) is usually made into simpler two-layer petits fours. A decent alternative to génoise for those who will accept nothing less than the traditional four-layer petit four is pound cake. Granted it’s far richer and denser than génoise, but its tight crumb gives it good sliceability.
I confess that yesterday I hunted around the mega-mart for a store-bought pound cake when I was forced to run out for a few quick staples (life is nuts this week). However since I couldn’t find one, I’ll be forced to be honest.