• Preheat the oven to 350-F and prepare a 9 x 13 baking dish by lining it with parchment paper so that the parchment paper comes up over the sides of the dish.
• Make the brown butter. Melt the butter in a saucepan on the stove over medium-low heat and just let it cook. Stir frequently. The butter will foam up, and after the foam subsides, the milk solids in the butter will have turned a nut-brown color and will smell like caramel. Remove the pan from the heat and let it cool.
• Whisk the flour, cinnamon, nutmeg, and baking powder in a bowl. Set it aside.
• Add the cooled (but not solid) brown butter to the bowl of a stand mixer and mix in the eggs until well combined.
• Add the brown sugar to the egg-butter mixture and mix until combined.
• Add the dry ingredients to the wet mixture, and mix well, scraping down the sides of the bowl as needed.
• Remove the bowl from the mixer, and with a wooden spoon or spatula, fold in the oatmeal and diced apples.
• Transfer the batter to the prepared pan and smooth it out a bit. Pop it in the oven for about 30-35 minutes.
• Cool the Brown Butter Apple Blondies and make the glaze.

To make the glaze

• Melt the butter and brown sugar in a small pan on the stove over medium heat. Simmer it until the brown sugar is just dissolved. Remove the pan from the stove and let the butter-sugar mixture cool a bit.
• Add the maple syrup, vanilla, and powdered sugar to the melted butter mixture until smooth and thick enough to drizzle. Thin it with a teaspoon of milk if it is too thick.
• Drizzle the brown sugar glaze over the top of the Brown Butter Apple Blondies, then cut and serve!