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" Preheat oven to 350 degrees. Butter a 12-by-17-by-1-inch rimmed baking sheet. Line bottom with parchment paper. Butter parchment, and dust with flour, tapping off any excess. Set sheet aside. Into a medium bowl, sift together flour, baking powder, and salt; set aside. In a measuring cup, combine milk and vanilla; set aside. In the bowl of an electric mixer fitted with the paddle attachment, cream butter until very smooth. With mixer on medium speed, add sugar in a slow, steady stream; beat until mixture is light and fluffy, about 3 minutes. Reduce speed to low. Add reserved flour mixture in three batches, alternating with reserved milk mixture, and starting and ending with flour. Mix until just combined; do not overmix. Set aside. In a clean bowl, beat egg whites until stiff but do not let dry peaks form. Fold one-third of the egg whites into batter to lighten, then gently fold in remaining whites in two batches. Scrape batter into prepared sheet; smooth top with an offset spatula. Bake in oven until cake is springy to the touch and a cake tester inserted in the center comes out clean, about 30 minutes. Transfer to a wire rack, and let cool 15 minutes. Loosen sides of cake with a small metal spatula or paring knife; invert onto a buttered wire rack. Peel off parchment. To prevent splitting, reinvert cake so top is facing up. Let cake cool completely."^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.delish.com/cooking/recipe-ideas/recipes/a19759/white-sheet-cake-recipe-mslo1012/?src=shelter_footer>