The classic dish of this kind is chocolate mousse. In its purest form, it is made by melting chocolate—a blend of cocoa butter, starchy cocoa particles, and finely ground sugar—at around 100ºF/38ºC, combining it with raw egg yolks, and combining this mixture with 3 to 4 times its volume of stiffly beaten egg whites (see
p. 112). The watery foam walls are thusaugmented with the thick, yolky chocolate, and much of the egg moisture is absorbed by the cocoa solids and sugar, which further thickens the bubble walls. While still warm, the mousse is spooned into serving dishes, and these are then refrigerated for several hours. As the mousse cools, the cocoa butter congeals, and the bubble walls become rigid enough to maintain the foam structure indefinitely. The chocolate thus strengthens the egg foam, and the foam spreads the stodgy chocolate mass into a gossamer structure that melts on the tongue.