Preheat oven to 325°. Beat butter and sugar at medium speed with a heavy-duty electric stand mixer until fluffy. Gradually add egg whites, one-third at a time, beating well after each addition.
Sift together cake flour and baking powder; gradually add to butter mixture alternately with 1 cup water, beginning and ending with flour mixture. Stir in vanilla. Pour batter into a greased and floured 13- x 9-inch pan.
Bake at 325° for 45 to 50 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan on a wire rack (about 1 hour). Remove from pan to a serving platter. Spread top and sides of cake with frosting.
I wanted a recipe for a 9x13 cake that would be higher (deeper) than most recipes produce - so because this recipe calls for slightly larger amounts of everything than most sheet cake recipes, plus 7 egg whites, I assumed it would be higher and lighter than previous recipes I've made. I made the recipe exactly as called for. It came out only slightly deeper than other recipes. The cake was fairly dense, not as light as I'd expected. The flavor was good. The buttercream frosting recipe at the end was ridiculous! Result was very soft and loose, but spreadable (too much milk) - AND it made enough for at least 2 cakes! It tasted good, but half the recipe would be enough for this cake, even it you frost it fairly thickly. I had to refrigerate the cake after frosting it to keep the frosting from sliding off the sides. I probably will not make this cake again - it was not bad, but will look for others that come out lighter and less dense.
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