Preheat your oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper, ensuring the paper hangs over the edges for easy removal later.

In a large mixing bowl, combine the softened unsalted butter, granulated sugar, and brown sugar. Beat the mixture together for about 2-3 minutes or until it’s light and fluffy.

Add in the large egg and the vanilla extract, continuing to mix until everything is fully combined and silky smooth.

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

Slowly incorporate the dry ingredients into the wet mixture, stirring gently until just combined.

Carefully fold in the finely diced apple, ensuring that it’s evenly dispersed throughout the batter.

Pour the batter into your prepared 8x8 pan, spreading it evenly with a spatula. In a small bowl, mix the additional granulated sugar and ground cinnamon for the topping, then sprinkle this blend generously over the batter.

Place your pan in the preheated oven and bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out clean.

Once baked, remove the pan from the oven and let the bars cool completely in the pan on a wire rack.

Once cool, lift them out using the parchment overhang and cut into squares for serving.