Preheat oven to 350 degrees. Grease a 9-inch round cake pan.
In a bowl, mix together flour, cocoa, sugar, baking soda and salt. Sift. In a separate bowl, whisk together the water, oil, vanilla and vinegar.
Whisk together wet and dry mixtures. If lumpy, whisk until smooth, or pour through a strainer into a bowl and break up lumps, pressing them through.
Mix again, stir in chips if you are using them, and pour into prepared pan. Tap edge of pan against the counter, or drop from 6 inches to the floor several times to pop air bubbles. Bake in preheated oven 25 to 30 minutes, or until top springs back when pressed gently. Cool before removing from pan. Dust with powdered sugar, or frost, if desired.