Preheat oven to 350 degrees.
Line a 9x13 baking dish with doubled aluminum foil and a few inch overhang on both ends. Butter or spray lined the dish with nonstick cooking spray. (I prefer using a metal rather than glass dish for this but use whichever you like)
In a large bowl, stir together brown sugar, melted butter, eggs, and vanilla.
In a separate bowl, stir together flour, baking powder, and salt. Add to brown sugar mixture and stir until combined. Stir in apples and pecans. Pour batter into prepared baking dish.
5. Bake in preheated oven for 40-50 minutes or until a toothpick inserted in the center comes out clean. Cool completely.
Meanwhile, prepare glaze. In a heavy bottomed 2-quart saucepan, bring brown sugar, butter, and milk to a boil over medium heat. Boil 1 minute, whisking constantly. Remove from heat.
Add vanilla and powdered sugar and using a hand-held electric mixer beat on medium speed until smooth. Pout immediately over cooled blondies and sprinkle with pecans.