Preheat oven to 350 °F. Grease and flour two 9-inch round cake pans.
In a large mixing bowl, combine flour, sugar, cocoa, baking soda and salt. Add water, oil, vinegar and vanilla. Beat with a spoon or wire whisk just until batter is smooth and ingredients are well-blended.
Pour batter into prepared cake pans. Bake for 35 minutes, or until wooden pick inserted into the center of the cake comes out clean. Let cool in pans 10 minutes before turning out onto wire racks to cool completely. Frost with chocolate frosting.