In a large mixing bowl combine brown sugar, pumpkin, the whole egg, egg whites, and oil. Beat with an electric mixer on medium speed until blended. Add flour, baking powder, cocoa powder, cinnamon, allspice, salt, and nutmeg. Beat on low speed until smooth. Stir in semisweet chocolate pieces.
Spray an 11x7x1-1/2-inch baking pan with nonstick coating. Pour batter into pan; spread evenly. Bake in a 350 degree F oven for 15 to 20 minutes or until a wooden toothpick inserted near the center comes out clean. Cool in pan on a wire rack. Cut into 2-inch squares. Makes 15 servings.