Sift flour, sugar, salt, soda, and cocoa together into a 9 x 13 inch ungreased cake pan. Make three wells. Pour oil into one well, vinegar into second, and vanilla into third well. Pour cold water over all, and stir well with fork.
Bake at 350 degrees F (175 degrees C) for 30 to 40 minutes, or until tooth pick inserted comes out clean. Frost with your favorite icing.