Four layers of light and airy sponge cake are stacked and assembled using a rich, caramel-based buttercream. Martha made this recipe on Martha Bakes episode 604.
Preheat oven to 350 degrees. Butter bottom and sides of two 9-by-2-inch round cake pans. Line pans with parchment paper rounds and butter again. Dust with flour; tap out excess. In a small bowl, sift together flour and cornstarch.
In the bowl of an electric mixer fitted with the whisk attachment, beat egg yolks, vanilla, and 3/4 cup sugar on high speed until thick and pale, about 3 minutes. Transfer egg-yolk mixture to a large bowl. Wash and dry mixer bowl and whisk attachment.
Combine egg whites and salt in mixer bowl, and whisk on medium speed until soft peaks form, about 1 1/2 minutes. With mixer running, slowly add remaining 6 tablespoons sugar; continue whisking until stiff and glossy, about 1 minute more.
Fold egg-white mixture into egg-yolk mixture. In three additions, fold reserved flour mixture into egg mixture. Divide batter between prepared pans, smoothing tops with an offset spatula. Bake until light golden brown and a cake tester inserted in the center comes out clean, about 20 minutes.
Transfer pans to wire racks to cool slightly; turn out cakes, remove parchment paper, and let cool completely.
Using a serrated knife, halve each cake horizontally to form 2 layers. Transfer 1 layer to a serving platter and spread 1 cup frosting evenly on top. Set another cake layer on top, and repeat process with more frosting and remaining cake layers. Spread 1 1/2 cups frosting over entire cake. Refrigerate until firm, about 30 minutes. Spread remaining 2 1/2 cups frosting over cake, smoothing the top and sides. Use a cake comb to give sides of cake a decorative finish, if desired. Serve immediately, or refrigerate up to 2 days; let cake come to room temperature before serving.
The layer cakes can be made up to 2 weeks in advance. Let cool completely before wrapping tightly in plastic wrap and freezing. Thaw completely at room temperature before using.