Preheat oven to 375. Prepare a 8" square glass dish with cooking spray; set aside. In a mixing bowl, sift together flour, sugar, cocoa powder, baking soda, and salt. Using a spoon, make three wells in dry ingredients. Pour oil into one, vinegar into another, and vanilla extract into the third. Slowly pour water over all. Mix on low speed until all is moistened (batter will be thin). Pour into prepared dish. Bake for 20 minutes, or until toothpick inserted in center comes out clean. When cake has cooled, spread with frosting.