Ingredients
1 tablespoon unsalted butter, for pan
1/2 cup all-purpose flour, plus more for pan
1/2 cup cornstarch
4 large eggs, separated
1 teaspoon pure vanilla extract
3/4 cup sugar
Pinch of salt
Preparation
Preheat oven to 350 degrees. Butter a 9-inch square baking dish. Line bottom with parchment paper; butter parchment. Dust with flour, and tap out excess; set aside. Sift together flour and cornstarch into a small bowl; set aside.
Put yolks, vanilla, and 1/2 cup sugar into the bowl of an electric mixer fitted with the whisk attachment. Beat on high speed until thick and pale, about 3 minutes. Transfer to a large bowl.
Put egg whites and salt into the clean bowl of an electric mixer fitted with the clean whisk attachment. Beat on medium speed until soft peaks form, about 3 minutes. With mixer running, add remaining 1/4 cup sugar in a slow, steady stream. Continue beating until stiff, glossy peaks form, 2 to 3 minutes.
Using a rubber spatula, fold egg white mixture into the yolk mixture. Fold in reserved flour mixture in 3 batches. Transfer batter to prepared pan; smooth top with an offset spatula.
Bake until a cake tester inserted into center comes out clean, about 25 minutes. Let cool completely in pan on a wire rack. Turn out cake onto rack. Cake can be wrapped well in plastic and stored at room temperature up to 2 days.
