Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.

In a large bowl, mix together melted butter and granulated sugar until well combined.

Add in the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

In a separate bowl, whisk together flour, cocoa powder, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the diced apples until evenly distributed.

Pour the batter into the prepared baking dish and spread evenly.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

While the brownies are baking, prepare the maple glaze by whisking together powdered sugar, maple syrup, and milk in a small bowl until smooth.

Adjust the consistency with more milk or powdered sugar as needed.

Remove the brownies from the oven and allow them to cool in the baking dish for about 10 minutes.

Once cooled, drizzle the maple glaze over the top of the brownies.

Cut into squares and serve warm or at room temperature.