Sponge cakes are all about the eggs. Some include just the egg whites (angel food), others just yolks (yolk sponge cakes), but many have some of both. Add citrus zest and juice to the latter category and you have Sunshine Cake. Richer than angel food and more golden, these airy, almost-fat-free cakes take care to make, but yield a refreshingly light and brightly flavored result. The key? You truly have to beat the heck out of the yolks to get enough air into them. Whip them on highest speed for a full four minutes, until the yolks change from deep gold to a pale yellow. Also important: Whip the whites to thick, hold-their-shape glossiness, but take care not to whip to dryness. And when you fold the flour in to the whole airy mass, do so with a gentle over/under motion, so as not to lose the volume.
Makes one 10-inch tube cake
1 cup cake flour, sifted three times
1 1/4 cups sugar, sifted five times
4 large egg yolks
8 large egg whites
1/2 tsp salt
1/2 tsp lemon extract
Zest of 1/2 fresh lemon
Zest of 1/2 fresh lime
Lemon-Lime Frosting Ingredients
1. Preheat oven to 350 degrees
2. Beat egg yolks at highest speed for 4 minutes until yolks are light yellow.
3. Wash beaters. Whip egg whites until frothy; add salt and continue beating until whites volume-ize, holding their shape , but are still moist.
4. Fold sugar, in three passes, into the egg whites, using a rubber spatula to incorporate in an over/under motion.
5. Fold in extract, citrus rind and whipped egg yolks.
6. Fold in flour, again using a rubber spatula to incorporate in an over/under motion.
7. Transfer batter to an ungreased 10-inch tube pan, leveling the surface of the batter with spatula.
8. Bake 40 to 45 minutes until lightly browned (use the toothpick-in-the-center test to make sure cake is done) Invert cake in tube pan to cool.
9. While cake cools, make frosting: Whip sugar with butter and citrus zest, adding citrus juice as you go, to the consistency you desire. Frost cooled cake.