Line a 9x13 inch metal pan with nonstick foil or regular foil. If using regular foil, spray lightly with cooking spray.
In a small bowl, stir together bran cereal, flours, baking powder and salt. Set aside.
In large mixing bowl, whisk together oil, egg and sugar. Stir in vanilla and water. Add cereal mixture, stirring until well combined. Stir in chocolate chips.
Spread evenly in the pan. Sprinkle nuts over the top (if using).
Bake at 350° F about 30 minutes or until golden brown.
Let cool completely, then lift from pan and cut into 16 or 24 bars or squares.