Heat oven to 375°F. Coat an 8-inch square baking pan with cooking spray. Whisk together yogurt, oil, honey, and egg whites until well-combines.
In a separate bowl, stir together whole-wheat and all-purpose flours, baking powder, and cinnamon. Stir in oats and dates. Fold wet ingredients into the dry until moist. Stir in apple.
Spread evenly in pan. Bake 30 minutes, or until firm to touch. Cool on wire rack. Cut into 8 squares; then cut each square in half diagonally.