Preheat oven to 350°.
Beat sugar, butter, and vanilla in a large bowl at medium speed of a mixer until well-blended (about 5 minutes). Add egg whites and egg, 1 at a time, beating well after each addition. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, cocoa, baking powder, and salt in a small bowl, stirring well with a whisk. Combine buttermilk, water, and baking soda. Add flour mixture to sugar mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Pour into a 9-inch round cake pan coated with cooking spray. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes; remove from pan. Cool completely on a wire rack. Spread glaze over cake.