Preheat your oven to 350 degrees and spray an 8 x 8 baking dish with non-stick spray (I use Trader Joe's coconut oil). Set aside.
In a large bowl (or the bowl of your stand mixer), add the butter, sugar, egg, and vanilla and beat until fluffy.
In another medium bowl, add the flour, salt, baking powder, baking soda, cinnamon, and nutmeg and whisk together.
Pour the dry mixture into the wet mixture and stir with a spatula until just blended.
Fold in the diced apples and peanut butter chips.
Spread the batter evenly into your prepared pan.
Sprinkle the light brown sugar over the top.
Bake for 35 minutes or until a toothpick inserted into the center comes out clean.
Let cool on a rack before slicing.