Step 1: Preheat oven to 350°F (175°C). Grease and flour an 8x8 inch baking pan, or line with parchment paper, leaving an overhang on two sides for easy removal.
Step 2: In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla extract until well combined.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, baking powder, cinnamon, nutmeg, and salt.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix. Gently fold in the finely diced apples.
Step 5: Pour the batter into the prepared baking pan and spread evenly.
Step 6: Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with moist crumbs (not wet batter).
Step 7: Let the brownies cool completely in the pan on a wire rack before making the glaze.
Step 8: For the maple glaze, whisk together powdered sugar, maple syrup, and 1 tablespoon of milk or cream in a small bowl until smooth. Add more milk, 1/2 teaspoon at a time, until the desired pourable consistency is reached. Stir in vanilla extract if using.
Step 9: Once brownies are completely cool, drizzle the maple glaze evenly over the top. Allow the glaze to set for about 15-20 minutes before slicing and serving.