Drain a can of black beans and rinse thoroughly until the water runs clear. Return the black beans back to the can and fill with water. Puree the beans and water. Add the puree to the dry mix along with the vanilla and extra cup of water. Stir to combine.
Pour the batter into a greased 9×13 pan. Bake for 25-30 minutes, rotating the pan around halfway through. When the brownies are finished they should be firm in the center and the edges will be slightly puffy and starting to pull away from the sides.
Let brownies cool completely then use a 2×2 in cookie cutter to cut into 24 squares.
Optional: 1 1/2 cup walnuts (will add extra calories, though!).