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Preheat oven to 325F.
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Sift flour and ½ cup sugar in a mixing bowl.
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In the bowl of a stand mixer fitted with a whisk, beat the egg whites on medium speed until frothy, about 1 minute. Add lemon zest and juice, cream of tartar, vanilla, and salt; continue beating until soft peaks form. Add the rest of the sugar with the mixer running.
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Increase speed to high and beat until firm peaks form. Sprinkle ⅓ of the flour mixture on the whites and fold to combine. Repeat with remaining flour mixture in 2 additions.
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Transfer batter to an angel food cake pan and run a knife through the center to remove bubbles. Bake until a tester inserted in the center comes out clean, about 45 minutes.
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Invert pan onto a wire rack and let cool completely. Run a knife around the sides and middle of the pan. Remove to a serving platter.