Preheat oven to 350 °F
Spray an 8 inch pan with non-stick cooking spray.
Mix the flour, cocoa powder, sugar, baking soda, and salt in the prepared pan.
Make 3 depressions in the dry ingredients, two small and one larger.
Pour vinegar in one of the smaller depressions, and vanilla in the other. Add the vegetable oil in the larger depression.
Pour water over the entire mixture and mix well, until smooth.
Bake on the middle oven rack for 35 minutes, or until you can insert a toothpick into the center and it comes out clean.
Allow cake to cool completely before topping with a dusting of powdered sugar or frosting.