Heat oven to 350 degrees. Spray 15x10x1-inch baking pan with nonstick cooking spray. In large bowl, combine sugar and 1/2 cup butter or margarine; beat until light and fluffy. Add molasses and egg; beat well.
In small bowl, combine all-purpose flour, whole-wheat flour, baking soda and ginger; blend well. Add to sugar mixture alternately with buttermilk; mix well. Stir in grated orange peel. Spread evenly in sprayed pan.
Bake at 350 degrees for 15 to 20 minutes or until toothpick inserted in the center comes out clean. Cool 30 minutes or until completely cooled.
In small bowl, combine all frosting ingredients, adding enough orange juice for desired spreading consistency; beat until smooth. Spread over cooled bars. Cut into bars. Makes 48 bars.
Note: To substitute for buttermilk, use 2 teaspoons vinegar or lemon juice plus skim milk to make 2/3 cup.