Preheat the oven to 350 degrees. Butter the bottom and sides of an 11-by-17-inch rimmed baking sheet and a 9-inch-diameter round cake pan. Line the bottom of each pan with parchment paper, and butter again. Flour the pans, and set aside. In a small bowl, sift together the flour and cornstarch; set aside.
In the bowl of an electric mixer fitted with the whisk attachment, beat egg yolks, vanilla, and 3/4 cup sugar on high until thick and pale, about 3 minutes. Transfer egg-yolk mixture to a large bowl. Wash and dry the mixer bowl and the whisk attachment.
Combine the egg whites and salt in the mixer bowl, and beat on medium speed until soft peaks form, about 1 1/2 minutes. With the mixer running, slowly add the remaining 6 tablespoons sugar. Continue beating until stiff and glossy, about 1 minute.
Fold the egg-white mixture into the egg-yolk mixture. In three additions, fold the reserved flour mixture into the egg mixture. Transfer two-thirds of the batter to the baking sheet, and smooth the top with an offset spatula. Transfer remaining batter to the round cake pan; smooth the top with the spatula. Bake until light golden brown and a cake tester inserted into the center comes out clean, about 20 minutes. Transfer the pans to wire racks to cool; turn out the cakes, remove the parchment paper, and wrap in plastic wrap until ready to use.
The cakes can be made ahead, cooled completely, and frozen for up to 2 weeks. Thaw completely at room temperature before using.