The basic hot soufflé has as its starting point a roux—a cooked mixture of flour and butter. This is cooled slightly and blended with egg yolks and savoury or sweet flavouring ingredients which are either already cooked or do not require much cooking. The result resembles a thick rich sauce. Stiffly beaten egg whites are then folded in. The mixture is baked in a high-sided dish. It rises mainly through simple expansion of the air in the egg foam.