Preheat your oven to 350°F (175°C). Lightly grease an 8x8-inch square baking pan or line it with parchment paper for easy removal.

In a large mixing bowl, combine the melted butter and brown sugar. Whisk them together until smooth and well combined.

Add the egg and vanilla extract to the butter mixture. Whisk vigorously for about one minute until the batter looks shiny and smooth.

In a separate medium bowl, whisk together the flour, cinnamon, nutmeg, baking powder, and salt.

Gradually pour the dry ingredients into the wet ingredients, stirring until just combined. Be careful not to overmix.

Gently fold in the diced apples until evenly distributed throughout the batter.

Spread the batter into the prepared baking pan and smooth the top into an even layer.

Bake for 25 to 30 minutes, until the edges are golden brown and pulling away from the sides of the pan. A toothpick inserted into the center should come out clean or with a few moist crumbs.

Let the blondies cool completely on a wire rack before cutting them into squares.