SPONGE CAKE

Steps:

• Preheat oven to 350 degrees. Butter two 9-inch round cake pans or two 12-by-17-inch rimmed baking sheets. Line bottoms with parchment; butter parchment and flour pans, tapping out excess flour. Whisk together egg yolks and 1 cup granulated sugar in a bowl set over a saucepan of simmering water until sugar has dissolved and mixture is warm, 3 to 4 minutes. Remove from heat and beat with a mixer on medium-high speed until pale and thick enough to form a ribbon that dissolves onto itself, 3 to 5 minutes. Beat in vanilla and salt; transfer to a large bowl.
• Beat egg whites until soft peaks form. Gradually add remaining 1/2 cup granulated sugar and beat on medium-high speed until stiff, glossy peaks form, about 2 minutes. Fold one third of egg whites into yolks, then gently fold in remaining whites. Sift flour over top and gently start to fold in. When nearly all is folded in, pour melted butter down side of bowl and fold just until incorporated and smooth.
• Divide batter among pans and bake until a toothpick inserted in centers comes out clean, about 25 minutes for rounds, or 15 minutes for sheets, rotating and switching racks halfway through. If baking in round pans, immediately invert cake and remove parchment, then reinvert and cool right side up. If using baking sheets and making a rolled cake, immediately invert cake onto a kitchen towel dusted with confectioner's sugar. Dust top of cake with confectioner's sugar and roll up in towel. Let cool completely, about 1 hour, before unrolling and spreading with filling.