Preheat the oven to 350 degrees F (175 degrees C).
Sift flour, sugar, cocoa powder, baking soda, and salt together in an 8x8 inch ungreased cake pan. Make 3 depressions in flour mixture; pour oil into one well, vinegar into second, and vanilla into third well. Pour water over all, then stir with a fork until well blended.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 40 minutes.