Preheat your oven to 350°F and grease a 9x13 inch baking pan. No parchment drama—just good old butter.
In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg. This is your dry mix.
In another bowl, combine the melted butter and brown sugar until smooth. Add in the eggs and vanilla, and stir till it's dreamy.
Add the dry mix to the wet mix in two parts, alternating with the buttermilk. Don’t overthink it—just mix until it's all friendly.
Fold in the diced apples like you're tucking them in for a nap.
Pour the batter into the pan and smooth it out. Try not to taste it (or do, I won’t tell).
Bake for 28–32 minutes, or until a toothpick comes out with just a crumb or two. Let it cool before slicing, if you have that kind of patience.