Preheat oven to 350°F (175°C) and line a 9x9-inch baking pan with parchment paper.
In a large bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth.
Add eggs and vanilla extract, mixing until combined.
In a separate bowl, whisk together flour, baking powder, salt, cinnamon, and nutmeg.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
Fold in diced apples.
Spread the batter evenly in the prepared baking pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Allow the blondies to cool completely before glazing.