White Sheet Cake

Preheat oven to 350 degrees. Butter and flour a 12-by-18-by-1-inch rimmed baking sheet, tapping out any excess flour; set aside. In a medium bowl, sift together flour, baking powder, and salt; set aside. In the bowl of an electric mixer fitted with the paddle attachment, beat butter and 2 cups sugar until light and fluffy, 3 to 4 minutes, scraping down sides of the bowl as needed. Beat in vanilla. With mixer on low speed, add flour mixture in three parts, alternating with the milk and beginning and ending with the flour; beat until just combined. Transfer mixture to a large bowl; set aside. In the clean bowl of an electric mixer fitted with the whisk attachment, beat egg whites on low speed until foamy. With mixer running, gradually add remaining 1/4 cup sugar; beat on high speed until stiff, glossy peaks form, about 4 minutes. Do not overbeat. Gently fold a third of the egg-white mixture into the butter-flour mixture until combined. Gently fold in remaining egg-white mixture. Transfer batter to prepared baking sheet and smooth batter evenly with a spatula or back of a spoon. Transfer to oven and bake until a cake tester inserted in the center comes out clean, about 35 minutes. Transfer to a wire rack and let cool 10 minutes. Invert cake onto rack, then reinvert and let cool completely.