Preheat oven to 350°F (175°C). Grease a baking pan.
Mix flour, baking powder, and salt in a bowl.
In another bowl, combine melted butter, brown sugar, eggs, and vanilla extract.
Fold in the dry ingredients into the wet mixture.
Add chopped apples, cinnamon, and nutmeg. Stir well.
Pour batter into the prepared pan. Bake for 25–30 minutes.
Cool brownies completely before glazing.
For the glaze, whisk powdered sugar, maple syrup, and milk until smooth.
Drizzle glaze over brownies. Let it set before serving.