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

In a large bowl, cream together ½ cup unsalted butter, ½ cup granulated sugar, and ½ cup brown sugar using an electric mixer for 2-3 minutes.

Add 1 large egg and 1 teaspoon vanilla extract to the mixture and mix until well combined.

In a separate bowl, whisk together 1 ½ cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon salt, 1 teaspoon ground cinnamon, and ¼ teaspoon ground nutmeg.

Gradually add the dry ingredients to the wet mixture, mixing until just combined. Carefully fold in the diced apple.

Pour the batter into the prepared pan and spread it evenly.

Mix 2 tablespoons granulated sugar and 1 teaspoon ground cinnamon for the topping, then sprinkle it over the batter.

Bake for 25-30 minutes until the edges are golden brown and a toothpick comes out clean from the center.

Let the bars cool completely in the pan before lifting them out using the parchment and slicing into squares.