1. Heat oven to 350 degrees . Coat a 12-cavity brownie pan (such as Wilton Brownie Bar Pan) with nonstick cooking spray.
2. Place nuts on a rimmed baking sheet. Bake at 350 degrees for 10 minutes. Cool slightly and rub nuts with fingers to remove skin. Chop nuts and reserve.
3. Meanwhile, in a large bowl, combine sugars, butter, eggs and vanilla. Whisk together flour, baking powder and salt; stir into sugar-and-butter mixture until dry ingredients are just moistened. Fold in apples and 3/4 cup of the chopped nuts.
4. Spoon batter into prepared brownie pan, about 1/3 cup into each cavity. Bake at 350 degrees for 30 minutes or until a toothpick inserted into center comes out clean. Cool on a wire rack for 30 minutes; turn out onto rack to cool completely.Maple Icing
5. In a large bowl, beat cream cheese until smooth. Gradually beat in sugar; add maple syrup and vanilla and beat until smooth.
6. Frost each blondie with a generous 2 tbsp of icing. Sprinkle with remaining 1/4 cup chopped hazelnuts.