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

In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and brown sugar.

Beat in the egg and vanilla extract into the creamed mixture until fully 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 wet ingredients, mixing gently until just combined.

Fold in the peeled and finely diced apple.

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

Mix the sugar and cinnamon for the topping, and sprinkle it over the batter.

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

Allow to cool fully in the pan on a wire rack before lifting out and cutting into squares.