Preheat the oven to 350 degrees F. Lightly coat a 15101-inch baking pan with nonstick cooking spray; set aside. In a large bowl, stir together all-purpose flour, whole wheat flour, granulated sugar, pecans (if using), baking powder, cinnamon, baking soda, and salt.
In a medium bowl, combine egg, pumpkin, canola oil, and milk. Add pumpkin mixture to flour mixture along with 1/3 cup chocolate pieces; stir just until combined. Spread the batter very evenly in the prepared pan. Sprinkle top with 2 tablespoons chocolate pieces. Bake about 25 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack. If desired, sprinkle lightly with powdered sugar. Cut into bars.