Today the phrase 'sponge cake' is a very broad term. It basically means a plain cake made from the four basic ingredients; sugar, butter, eggs and flour.
One might believe that there are millions of different types of cakes and methods of making cakes. But actually there are three main methods of making cakes that form the basis of a range of different flavours and textures of cakes. By tweaking the amount of one ingredient in a recipe or adding an extra one (e.g. flavouring) the outcome of the cake can be completely changed. The three main methods are CREAMING, FOAMING and LIQUEFYING (as I like to call it!).
Creaming involves creaming the butter and sugar together. It produces a dense cakes like a pound cake or biscuits. The creaming process pushes the sugar crystals through the fat (butter) forming a tunnel of air. This process breaks down the sugar until it is eventually dissolved into the butter. The tunnels of air expand during cooking and therefore the cake rises.
Foaming involves whipping the sugar and eggs together creating a foam; hence the name! Foaming produces lighter and airier cakes like a genoise sponge. Eggs have a very special protein structure called albumin, which is viscous yet stretchy. When the eggs are whipped the albumin is loosened and stretched into a web strong enough to hold air bubbles in it. The more the eggs are whipped the more air bubbles that are incorporated. When the mixture is cooked the air bubbles expand with the heat which causes the rising of the cake.
Liquefying is the method which doesn't use a creaming or foaming method. This method produces a type of cake called a Financier. Here the egg yolks are emitted and powdered sugar is used. Although the egg white has the same amount of stabilising proteins as the egg yolks they contain a higher percentage of water. Therefore when the whites are whipped they will incorporate air but won't foam and will stay as a liquid. The butter is melted and added at the end. Powdered sugar is used as it can dissolve more easily without the need for creaming or whipping.
The master ratio for making these cakes is half liquid to half solid. But watch out as butter and sugar are classed as liquids and flour and eggs are classed as solids. This is because when sugar is heated up it turns into a liquid and butter is also a liquid when warmed; therefore when the batter is cooked the butter and sugar melt and are absorbed by the solids. Whereas eggs are liquids at room temperature but when they are heated they solidify. Also they have a very strong structure, as they are packed with proteins.