Preheat oven to 375°F. Sift together flour, baking cocoa, soda, salt and sugar into a 9 inch round or 8 inch square pan.
In a bowl, blend the oil, water/coffee and vanilla and pour into the pan. Whisk batter until smooth. Add vinegar and stir quickly. (There will be swirls in the batter as the vinegar and soda mix and react.) Stir until vinegar is evenly distributed.