Preheat the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, letting excess hang over the edges.

In a large mixing bowl, cream the softened butter, granulated sugar, and brown sugar on medium speed for 2-3 minutes until light and fluffy.

Add the egg and vanilla extract, mixing on low speed until combined and smooth, about 1-2 minutes.

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

Gradually add the dry mixture to the wet ingredients, mixing just until no dry flour is visible.

Fold in the diced apple, ensuring they are evenly distributed throughout the batter.

Spread the batter into the prepared baking pan, smoothing the top evenly.

Mix together a bit of sugar and cinnamon for the topping, and sprinkle it over the batter.

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

Cool completely in the pan on a wire rack, then lift out and slice into squares for serving.