Brown All the Butter:
Place all 20 tablespoons of butter (for both blondies and icing) in a light-colored skillet over medium heat. Cook, stirring constantly, for 5-8 minutes, until the butter melts, foams, and develops browned specks at the bottom with a nutty aroma. Immediately pour the browned butter into a heat-safe bowl, then set aside 3 tablespoons for the icing.

Prepare the Apples and Blondie Batter:
Preheat your oven to 350°F (177°C) and line a 9×13-inch pan. In the same skillet, cook the chopped apples with maple syrup and cinnamon over medium heat for 3-5 minutes, until slightly softened. In a large bowl, whisk together the flour, baking powder, salt, and spices. In a separate bowl, whisk the larger portion of brown butter with the brown sugar, eggs, and vanilla. Pour the wet ingredients into the dry, mix until just combined, then fold in the cooked apples.

Bake the Blondies:
Spread the batter evenly into the prepared pan. Bake for about 35 minutes, until the top is lightly browned and a toothpick inserted into the center comes out with moist crumbs attached. Let the blondies cool completely in the pan.

Make the Icing and Finish:
To make the icing, whisk the reserved 3 tablespoons of brown butter with the confectioners’ sugar, milk, vanilla, and salt until smooth. Drizzle or spread the icing over the completely cooled blondies. Let the icing set before slicing.