Learn to make this ethereal European sponge cake and you'll be rewarded with a repertoire of outstanding desserts.
Coat a 12-by-16-inch rimmed baking sheet or a 9-inch round cake pan with cooking spray. Line with parchment; coat with cooking spray. Dust with flour, tapping out excess. Stir together flour and salt into a medium bowl; set aside. Preheat oven to 450 degrees for sheet cake or 400 degrees for round cake, with rack in lower third.
Put eggs, egg yolks, and granulated sugar in the heatproof bowl of an electric mixer. Set over a pan of simmering water; whisk until sugar has dissolved and mixture is warm to the touch. Return bowl to mixer fitted with whisk attachment. Beat on medium-high speed for 2 minutes. Raise speed to high. Beat until mixture is pale and thick, about 4 minutes more.
Remove bowl from mixer. Sift flour mixture over the top; using a large rubber spatula, carefully fold into egg mixture, cutting through middle and lifting gently over sides. When almost incorporated, pour melted butter down side of bowl; gently fold to incorporate completely.
Spread batter evenly in sheet or pan with a large offset spatula. Bake until golden brown and springy to the touch, about 6 minutes for 12-by-16-inch cake and 15 minutes for 9-inch round cake.
For the 12-by-16-inch cake: Run a small knife around sides of sheet; invert onto a wire rack. Remove parchment. For the roll, transfer cake to a dish towel dusted with confectioners' sugar, and roll up, starting at a short end; for the torte, reinvert cake onto the rack. Let cool completely. For the 9-inch round cake: Let cool in pan on a wire rack for 15 minutes; run a small knife around sides of pan. Invert onto rack; remove parchment, and reinvert onto rack. Let cool completely.
You can bake the genoise the day before you fill it. Wrap the cooled cake in plastic, and keep at room temperature.