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Genoise, a foam type cake, is an Italian cake named after Italy's city of Genoa. It is one of the most useful cakes because it is firm and sturdy. Because of its plain crumb, it is light and delicious on its own; the ways in which it can be decorated are myriad. It is often cut into layers and brushed with a flavored syrup or spirits, and layered with jams, buttercream, mousses, whipped ganache, pastry cream and then fruit and other fillings. it makes a great foundation cake for both elaborate or simple concoctions, such as wedding cakes, layer cakes, tortes, ice cream cakes, Baked Alaska, ladyfingers, petits fours and simpler desserts.
The genoise differs from the traditional American sponge cake, as well as the classic French biscuit, in that whole eggs are gently heated with sugar and whisked until they are foamy, slightly pale, and reads 110 to 120 degrees F with an Instant Read Thermometer. The heated egg foam passes through various stages as it is beaten - it first becomes foamy, then light and aerated and finally it thickens until it forms a thick ribbon when lifted from the bowl, called ribboning. The protein in the egg foam becomes partially coagulated from the heat, transforming it into an elastic mass. As it is beaten, the recipe holds large volumes of air, which, in turn, results in a batter with high volume and a cake that bakes lighter and higher.
SARAH SAYS: Warming the egg-and-sugar ingredients simply helps dissolve the sugar better and improves the emulsifying properties of the eggs. As a result, it helps the eggs reach maximum volume when beaten. Make sure the sugar is dissolved by rubbing a small amount of mixture in between your fingers. It should not be sandy. If it is, stir rapidly until the sugar is thoroughly dissolved. I like to use superfine sugar because it dissolves faster than regular table sugar. It's very important because if the sugar is NOT dissolved all the way, the egg structure won't be strong.
Once the egg-and-sugar foam has reached maximum volume or ribbons properly, sifted flour (with any dry flavorings) is folded in several additions so as not to deflate it. Optionally, melted and clarified butter is folded in at the very end, too. The cake contains no chemical leavener, such as baking powder or baking soda and is not normally flavored, except for a small amount of vanilla. During baking, the cake rises when the air trapped in the whipped eggs and the air produced as the water in the butter turns to steam, expands.
The Genoise is different from American sponge cakes in that it has less sugar and sometimes contains clarified butter (butter that has been melted so that the water evaporates and the milk solids drop to the bottom. The milk solids then brown which adds a richer flavor). The butter makes the cake somewhat more moist and flavorful. It tends to be dry and usually some type of syrup or icing is added to the finished cake in order to moisten it.
Butter Variation: You can brown the butter (buerre noisette) or clarify it for use in the recipe.