Chiffon cake is one of those classic cakes that is perenially popular, perhaps because it has such a singular texture that's created by having beaten egg whites folded into the batter. This kind of cake is always made with oil rather than butter, too, for a softer crumb and more delicate taste. Orange is one the more classic flavors, as in this recipe, where the zest and juice are added. To test for doneness, insert a toothpick into the middle; it shold come out clean.