Brown the butter over medium heat.
Add apples, maple syrup/brown sugar, and cinnamon over medium heat and cook 3-5 minutes until apples have slightly softened. Set aside.
Preheat oven to 350 degrees. Line 9x13 baking with with parchment paper.
Make the blondies: Whisk flour, baking powder, salt, cinnamon, and nutmeg together in a large bowl.
In a medium bowl, whisk 1 cup brown butter, brown sugar, eggs, and vanilla together. Pour the wet ingredients into the dry ingredients and stir until combined. Fold in the apples.
Evenly spread batter into pan. Bake for about 35 minutes or until top is lightly browned and a toothpick comes out clean. Remove from the oven and allow the blondies to cool completely.
Make the brown butter icing: whisk together 1/4 cup brown butter, confectioners sugar, milk, and vanilla extract. Drizzle over cooled blondies, then cut into squares.
Cover and store leftover iced blondies at room temperature for up to 2 days or in the refrigerator for up to 1 week.