These sponge cake cookies can be found in most supermarkets, but none will top the flavor of this homemade version. Martha made this recipe on Martha Bakes episode 604.
Preheat oven to 400 degrees. Using a pencil, mark two sheets of parchment with 3-inch lines; place upside-down on baking sheets. In a small bowl, sift together flour and cornstarch.
In the bowl of a standing mixer fitted with the whisk attachment, whisk egg yolks, vanilla, and 6 tablespoons 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 remaining 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 3 tablespoons sugar. Continue whisking until stiff and glossy, 2 to 3 minutes more.
Fold egg-white mixture into egg-yolk mixture. In three additions, fold reserved flour mixture into egg mixture. Transfer batter to a pastry bag fitted with a 5/8-inch tip. Holding bag horizontally, pipe 30 ladyfingers between the two baking sheets, using the 3-inch marks as a guide. Bake until deeply golden, 11 to 13 minutes. Let cool on wire rack.