Preheat oven to 350°F/175°C. Lightly grease an 8-inch square pan or a 9” round pan that is at least 2 inches deep.
Add the flour, sugar, cocoa, salt, and baking soda directly to the pan. Use a fork or a whisk to mix it all together, until well combined.
Create three indentations or wells in the dry ingredients. Add vanilla to the first well, vinegar to second, and oil in the third. Then pour the water over everything in the pan. Use a fork to stir the ingredients together until well blended.
Bake for 30-35 minutes, until a toothpick inserted in the center of the cake comes out clean, or with a few moist crumbs. Serve from the pan.
For the optional frosting: Gradually heat chocolate chips with the half and half, stirring occasionally, until chocolate is melted and smooth. Pour over cooled cake.
Cover cake and store at room temperature for several days, or freeze.