Butter

Even though butter adds richness and prolongs the life of the cake, its primary use in sponge cakes is as flavoring. In order to keep the foamy batter smooth, the only way to add butter is in liquid form. It is gently folded in after being melted but not when it is too hot lest it deflate the foamy mixture in anyway. You may also use hazelnut, almond or walnut oils if you want to perfume the cake with their unique flavors.