In a bowl, cream butter and sugar. Beat in water. Combine the flour, cocoa, baking soda and salt; add to creamed mixture and mix well. Add vanilla. In another bowl, beat egg whites on medium speed until soft peaks form. Gently fold into the creamed mixture.
Pour into a 9-in. square baking pan coated with cooking spray. Bake at 350° for 40-45 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack. Dust with confectioners' sugar.