In an ungreased square baking pan (8x8x2-inches), mix flour, sugar, cocoa, baking soda, and salt with a fork. Stir in water, vegetable oil, vinegar, and vanilla extract. Sprinkle chocolate chips over the batter, if desired.
Bake 35-40 minutes, or until a toothpick inserted near the center tests clean. Cool in pan on a wire rack.
Sprinkle with powdered sugar before serving, if desired.
Notes
This cake can be stored airtight at room temperature for up to 2-3 days. For longer storage, refrigerate in an airtight container for up to 5-7 days.