- Heat oven to 375 °F.
- Grease a 9-inch square baking pan.
- In large bowl with electric mixer, beat brown sugar and butter until creamy.
- Add eggs; beat until light and fluffy.
- Combine both flours, the cinnamon, baking powder, baking soda, and salt.
- Stir into butter mixture until blended.
- Stir in apples, oats, and nuts.
- Spread mixture in pan and bake 25 to 30 minutes or until wooden pick inserted in center comes out clean.
- Cool completely.
- If desired, top with icing and garnish with nuts before cutting into bars.