Beat egg whites in large bowl on high speed of mixer until soft peaks form; gradually add 1/2 cup sugar, beating until stiff peaks form. Beat egg yolks and vanilla in medium bowl on medium speed 3 minutes. Gradually add remaining 1/3 cup sugar; continue beating 2 additional minutes. 3. Stir together flour, cocoa, baking powder, baking soda, and salt; on low speed, add to egg yolk mixture alternately with water, beating just until batter is smooth. Using rubber spatula, gradually fold chocolate mixture into beaten egg whites until well blended. Spread batter evenly in prepared pan.