Sweet Potato Cake

This wonderful cake is made with olive oil instead of butter. It is light and moist and keeps well. Make sure all of your ingredients are at room temperature before you begin. If your mixer has only one bowl, beat the egg whites first, then turn them out into another bowl — preferably copper— while you continue with the dish. Use a 10-inch tube pan for this cake, but don’t grease it: you want the batter to cling to the sides and rise high. Nonstick pans with removable inserts work particula