Adapted from marthastewart.com
Vegetable oil cooking spray, for sheet and parchment
large egg whites, room temperature
teaspoon cream of tartar
cups superfine sugar
cup all-purpose flour
1. Preheat oven to 350 degrees. Coat a 13-by-17-inch rimmed baking sheet with cooking spray, and line with parchment. Coat parchment with spray. 2. Whisk whites and cream of tartar with a mixer on medium speed until soft peaks form. Slowly add 3/4 cup sugar, and whisk until thick and glossy, about 3 minutes. 3. Sift remaining 1/2 cup sugar, the flour, and a pinch of salt over egg-white mixture, and fold gently until just combined, being careful not to deflate whites. Spread batter evenly onto prepared baking sheet. Bake until golden and set, about 20 minutes. 4. Let cool completely in sheet on a wire rack. (Cake should pull away from sides easily.) Slide cake out from sheet, right side up; cut in half crosswise to form two 8 1/2-by-13-inch pieces. Carefully remove parchment from cake. Cook's Note Storage: Cake can be stored at room temperature, wrapped in plastic, for up to 2 days.