Preheat oven to 350°.
Combine sugar, flour, salt, and baking soda in large bowl. Sift in cocoa powder, and whisk all together. Make a well in the center of the dry ingredients; add oil, vanilla, vinegar, and water. Mix until thoroughly combined into a smooth batter.
Pour into a greased 9 x 13 pan and bake 30 minutes, or until toothpick inserted in center of cake comes out clean. Cool completely, then frost.