Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, combine the softened unsalted butter, granulated sugar, and brown sugar. Beat the mixture on medium speed for about 2–3 minutes until it turns light and fluffy.

Add the egg and vanilla extract to the mix. Beat together until fully incorporated.

In a separate bowl, whisk together the all-purpose flour, baking powder, salt, ground cinnamon, and nutmeg.

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

Fold in the finely diced apple, ensuring even distribution throughout the batter.

Spread the batter into the prepared baking pan and sprinkle the topping mixture over the batter.

Bake for 25–30 minutes until the edges turn golden and a toothpick inserted in the center comes out clean.

Let the bars cool completely in the pan before lifting out and cutting into squares.