Brown 2 1/2 sticks of butter (1 1/4 cups). To brown the butter melt it in a small saucepan over medium heat, stirring constantly. The butter will foam once melted, continue stirring. After 5-7 minutes the butter will begin browning and have a nutty aroma. Immediately remove from heat and pour into heat proof bowl.Once it's browned reserve 1/4 cup of it for the icing. The rest will be used in the blonde batter. Preheat the oven to 350. Line a 9x13 pan with parchment paper and lightly spray with cooking spray.
Peel and chop the apples into bite sized pieces, add them to the same pan you browned the butter in along with the brown sugar and cinnamon. Cook and stir over medium heat until the apples start to soften, about 5 minutes.
Whisk the flour, baking powder, salt, cinnamon, and nutmeg together in a large bowl. In another bowl whisk the 1 cup browned butter, eggs, brown sugar and vanilla together. Pour into the dry ingredients and stir to combine. Stir in the apples.
Spread the batter into the pan. Bake for 30- 35 minutes, or until a toothpick comes out mostly clean. Let cool completely.
To make the icing heat the 1/4 cup brown butter in the microwave to soften for a few seconds. Whisk in the maple syrup, cinnamon, vanilla, powdered sugar, and salt. Spread on cooled blondies. Enjoy!