Preheat your oven to 350°F (175°C) while you prepare the baking pan. Line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, combine the softened unsalted butter, granulated sugar, and brown sugar. Cream for about 2-3 minutes until light and fluffy.

Crack in one large egg and pour in the vanilla extract, mixing on low speed until well combined.

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

Gradually add the dry mixture to the creamed butter and sugar mixture, stirring gently with a spatula until just combined.

Fold in the finely diced medium apple, ensuring it's evenly distributed throughout the batter.

Transfer the batter into the prepared baking pan, spreading it evenly using a spatula.

Mix together some granulated sugar and ground cinnamon for the topping, and sprinkle it over the batter.

Place the pan in the preheated oven and bake for 25-30 minutes. Check for doneness with a toothpick.

Once baked, allow the bars to cool completely in the pan on a wire rack before cutting into squares.