Preheat your oven to 350 degrees F. Grease a 9x13 inch baking pan with butter or nonstick spray and set it aside.

Melt the butter in a light-colored saucepan over medium heat, stirring frequently. It will foam, then the foam will settle, and small golden-brown bits will form on the bottom. The butter should smell nutty and toasty, which takes about 5 to 7 minutes. Pour it immediately into a large mixing bowl and let it cool until it is warm to the touch but not hot, about 15 minutes. Do not skip this cooling step or you will scramble your eggs.

Whisk the brown sugar and granulated sugar into the cooled brown butter until combined. Add the eggs one at a time, whisking well after each one. Stir in the vanilla extract.

Add the flour, baking powder, baking soda, cinnamon, nutmeg, and salt to the bowl. Switch to a spatula and fold until just combined, stopping when you no longer see dry streaks of flour. Do not overmix.

Fold the diced apples into the batter gently. The batter will be thick and that is exactly right.

Spread the batter evenly into the prepared pan using your spatula. It helps to press it into the corners with lightly dampened fingertips.

Bake for 28 to 32 minutes, until the top is golden brown and a toothpick inserted in the center comes out with just a few moist crumbs. If it comes out wet with raw batter, give it another 3 to 4 minutes.

Cool the pan completely on a wire rack before cutting, at least 1 hour. Cutting warm blondies leads to crumbling edges. Once fully cooled, cut into 24 squares and serve.