Preheat oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal. Lightly grease any exposed pan.

In a large bowl, whisk the melted butter and brown sugar together until smooth.

Add the eggs and vanilla extract to the butter mixture. Whisk vigorously for about 1 minute until the mixture is thick and glossy.

In a separate bowl, whisk together the flour, salt, baking powder, cinnamon, and nutmeg.

Add the dry ingredients to the wet ingredients. Use a spatula to fold until just combined and no flour pockets remain. Do not overmix.

Gently fold the finely diced apples into the thick batter until evenly distributed.

Spread the batter evenly into the prepared pan, pressing it smoothly into the corners.

Bake for 30-35 minutes, until the edges are golden and the center is set but still slightly soft. A toothpick should have a few moist crumbs.

Let the blondies cool completely in the pan on a wire rack. For clean cuts, refrigerate after cooling.

For the glaze: Whisk all glaze ingredients, starting with 1 tbsp of milk, until smooth. Add more milk until a drizzle-able consistency is reached.

Lift the cooled blondies from the pan using the parchment paper. Slice into bars, drizzle with glaze if desired, and serve.