Génoise is the foundation of many layered desserts, like Pierre Hermé's famous cakes. Sometimes bakeries or restaurants will label any yellow or sponge cake a génoise, but an authentic génoise is distinguished by the technique used to prepare it. Most sponges will require the egg whites and yolks to be whipped separately until they increase in volume. Then they are gently folded together, along with the dry ingredients.
Not so with a génoise. The yolks, whites and some sugar are warmed over a water bath until they reach a high temperature - about 110° F. Then they are whipped with an electric mixer until they dramatically increase in volume. The first time I made one I was shocked! It nearly overflowed out of my bowl. The volume of the eggs and the steam produced by the butter create a high, pale cake that is dense and sometimes a little dry. But this makes it an excellent foundation for a cake that's going to be slathered with wet fruit topping or soaked in syrup overnight.
Génoise is the workhorse of the cake kitchen: it takes just a few basic ingredients to turn out a cake with a tender yet firm crumb that can support custard fillings, mousses and ganache.