Preheat the oven to 180 degrees. Grease a round or square pan and line with parchment paper.
Sift together the flour, cocoa, soda, salt, and sugar directly in a medium bowl, tip in any left over granules. In a 2 cup measuring cup or small bowl, measure and mix together the oil, cold water or coffee, and vanilla.
Pour the liquid ingredients into the dry ingredients and mix with a fork or a small whisk. When the batter is smooth, add the vinegar and stir quickly. There will be pale swirls in the batter as the baking soda and vinegar react. Stir just until the vinegar is evenly distributed throughout the batter and put it in the oven right away. Bake for 25 to 30 minutes and set aside to cool.