1. Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking pan or line it with parchment paper for easy removal.
2. In a large mixing bowl, combine the melted butter and brown sugar. Stir until the mixture is well combined and smooth.
3. Add the eggs and vanilla extract to the butter-sugar mixture, and beat until fully incorporated. This should take about 1-2 minutes for the mixture to become slightly frothy.
4. In a separate bowl, whisk together the flour, baking powder, salt, cinnamon, and nutmeg. Gently fold the dry ingredients into the wet mixture using a spatula or wooden spoon.
5. Fold in the diced apples and walnuts (if using), ensuring they are evenly distributed throughout the batter.
6. Pour the batter into the prepared baking pan, spreading it evenly to the edges.
7. Bake for 25-30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
8. Allow the blondies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely. This will help them set and make cutting easier.

From my experience, letting the blondies cool completely before slicing ensures they hold their shape beautifully. Patience is key here!