In a large saucepan, bring water to a boil. Remove from the heat. Stir in sugar and butter until butter is melted. Add eggs and vanilla; mix well. Combine flour, cocoa, baking soda and salt; add to butter mixture and mix thoroughly.
Pour into a greased 15x10x1-in. baking pan. Bake at 350° for 30-35 minutes or until a toothpick inserted in the center comes out clean. Cool completely on a wire rack.
For frosting, beat butter, confectioners' sugar, cocoa, vanilla and enough milk to reach a spreading consistency. Frost cake.