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Preheat oven to 350°F with rack in middle.
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Sift together flour, cocoa powder, 3/4 cup sugar, and 1/4 tsp salt.
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Beat whites with lemon juice, vanilla, and remaining 1/4 tsp salt using an electric mixer on medium-high speed until they just hold soft peaks. With mixer on high speed, add remaining 3/4 cup sugar in a slow stream and beat until whites hold stiff, glossy peaks, 3 to 5 minutes.
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Sift flour mixture over whites and beat on low speed until just blended (folding in any unblended flour mixture by hand if necessary).
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Spoon batter into ungreased tube pan and smooth top. Run a rubber spatula or long knife through batter to eliminate any large air bubbles.
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Bake until a wooden pick inserted into middle of cake comes out clean, 40 to 45 minutes. Remove from oven and immediately invert pan. If pan has “legs,” stand it on those. Otherwise, place pan over neck of a wine bottle. Cool cake completely, upside down, 1 to 1 1/2 hours. Turn pan right side up. Run a knife around edge and center tube of pan. Lift cake, still on bottom of pan, then run a knife under bottom of cake to loosen. Invert to release cake from tube, then reinvert onto a plate. Dust lightly with cocoa powder.