2. Put egg whites in the clean bowl of a standing mixer and whisk on medium-low speed until frothy, about 2 minutes. Add cream of tartar, increase speed to medium-high, and continue whisking until stiff peaks form, 2-3 minutes. Carefully fold one-third of the whites into yolk mixture with a rubber spatula, then fold in remaining whites. Sift about a quarter of the flour over egg mixture, then gently fold it in. Continue to sift and fold until flour is incorporated. Pour into prepared pan and place in oven. Lower heat to 325° and bake until golden, about 30 minutes. Cool in pan for 5 minutes, then unmold, peel off parchment paper, and cool on a rack.