Preheat oven to 350°F. Generously grease an 8-inch by 8-inch glass or metal baking pan.
Combine flour, brown sugar, cocoa powder, baking soda and salt in a medium bowl.
Combine unfed discarded starter, water, vinegar, vanilla extract and olive oil in a separate small bowl.
Add wet ingredients to dry ingredients. Stir until moistened and pour batter into prepared pan.
Bake for 25 to 30 minutes or until a fork inserted in the center of the cake comes out clean. Cool the pan on a baking rack.
If frosting the cake, allow it to cool completely first.