In a large bowl, use an electric mixer to beat egg yolks with 1/2 cup sugar until thick and pale. Blend in 1/3 cup cocoa, 1 1/2 teaspoons vanilla, and salt. In large glass bowl, using clean beaters, whip egg whites to soft peaks. Gradually add 1/4 cup sugar, and beat until whites form stiff peaks. Immediately fold the yolk mixture into the whites. Spread the batter evenly into the prepared pan.