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.
Several people have commented that this cake is too dry. I shortened the cooking time by 5 minutes, which helped keep it moist. The cake has nice taste, although not as rich as fudge brownies or a typical chocolate layer cake. I served it with ice cream and raspberry sauce.
Many of the other reviewers commented on the dryness of this cake. I used the required cake flour, knowing it would provide some lightness. However, this was just an awfully dry recipe. I used a different glaze on it but that didn't help at all. I won't make it again.
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