PreparationMake cake: Put oven rack in middle position and preheat oven to 325°F. Butter sides of a 17- by 12- by 1-inch baking pan and line bottom with parchment paper. 3Sift flour with salt into a bowl. Beat egg whites in a large bowl with an electric mixer at high speed until they just hold soft peaks, then add granulated sugar, 1 tablespoon at a time, beating, and continue to beat until whites hold stiff, glossy peaks, 3 to 4 minutes in a stand mixer or 4 to 6 with a handheld. Add yolks and vanilla and beat until very thick, about 2 minutes. Sift flour mixture into egg mixture in 3 batches, folding in each batch gently but thoroughly. Spread batter evenly in lined baking pan. Bake until top of cake is pale golden and springs back when pressed lightly, 5 to 18 minutes. Cool cake in pan on a rack 5 minutes, then run a small knife around edges to loosen. Lightly dust top with confectioners sugar. Invert pan onto kitchen towel, then remove pan and peel off parchment. Cool cake completely, about 15 minutes. Make pastry cream while cake bakes and cools: Whisk together cornstarch and milk in a 1-quart heavy saucepan until smooth. Bring to a boil over moderate heat, whisking constantly, then boil, whisking constantly, 1 minute. Whisk milk mixture gradually into egg yolk in a small bowl. Return mixture to saucepan and cook over moderate heat, whisking constantly, 1 minute. Transfer to a metal bowl, then put bowl into a larger bowl of ice and cold water and cool pastry cream, stirring occasionally, to room temperature, 6 to 8 minutes. Beat butter with confectioners sugar (1/4 cup) and vanilla in a medium bowl with cleaned beaters at medium speed until pale and fluffy, about 2 minutes. Beat cooled pastry cream, 1 tablespoon at a time, into butter mixture until smooth. Assemble and ice cake: Halve cake lengthwise, then transfer half to a cutting board and spread with all of pastry cream. Top with other half.
PreparationMake cake: Put oven rack in middle position and preheat oven to 325°F. Butter sides of a 17- by 12- by 1-inch baking pan and line bottom with parchment paper. 3Sift flour with salt into a bowl. Beat egg whites in a large bowl with an electric mixer at high speed until they just hold soft peaks, then add granulated sugar, 1 tablespoon at a time, beating, and continue to beat until whites hold stiff, glossy peaks, 3 to 4 minutes in a stand mixer or 4 to 6 with a handheld. Add yolks and vanilla and beat until very thick, about 2 minutes. Sift flour mixture into egg mixture in 3 batches, folding in each batch gently but thoroughly. Spread batter evenly in lined baking pan. Bake until top of cake is pale golden and springs back when pressed lightly, 5 to 18 minutes. Cool cake in pan on a rack 5 minutes, then run a small knife around edges to loosen. Lightly dust top with confectioners sugar. Invert pan onto kitchen towel, then remove pan and peel off parchment. Cool cake completely, about 15 minutes. Make pastry cream while cake bakes and cools: Whisk together cornstarch and milk in a 1-quart heavy saucepan until smooth. Bring to a boil over moderate heat, whisking constantly, then boil, whisking constantly, 1 minute. Whisk milk mixture gradually into egg yolk in a small bowl. Return mixture to saucepan and cook over moderate heat, whisking constantly, 1 minute. Transfer to a metal bowl, then put bowl into a larger bowl of ice and cold water and cool pastry cream, stirring occasionally, to room temperature, 6 to 8 minutes. Beat butter with confectioners sugar (1/4 cup) and vanilla in a medium bowl with cleaned beaters at medium speed until pale and fluffy, about 2 minutes. Beat cooled pastry cream, 1 tablespoon at a time, into butter mixture until smooth. Assemble and ice cake: Halve cake lengthwise, then transfer half to a cutting board and spread with all of pastry cream. Top with other half.