Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper for easy removal.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
In a large bowl, combine the melted butter, brown sugar, and granulated sugar. Mix until smooth.
Add the eggs and vanilla extract to the butter mixture, stirring until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Fold in the diced apples, and if using, the nuts and raisins or dried cranberries.
Pour the batter into the prepared baking pan, spreading it evenly.
Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Allow the brownies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely. Cut into squares and serve.