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.

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!