Preheat your oven to 350°F (175°C). This ensures that the cake bakes evenly.

Grease a 9-inch round cake pan with butter or non-stick spray and line the bottom with parchment paper for easy removal.

In a large mixing bowl, combine the sifted flour, granulated sugar, brown sugar, baking powder, baking soda, ground cinnamon, and salt. Whisk together until well combined.

In a separate bowl, whisk together the melted butter, eggs, and vanilla extract until smooth. This will help incorporate air into the batter, making the cake light.

Pour the wet ingredients into the dry ingredients and stir gently with a spatula until just combined. Be careful not to overmix, as this can lead to a dense cake.

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

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

In a small bowl, mix together the granulated sugar and ground cinnamon for the topping. Sprinkle this mixture evenly over the batter.

If using, sprinkle the chopped walnuts on top of the cinnamon sugar.

Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted into the center comes out clean. This ensures that the cake is fully cooked.

Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes.

Carefully run a knife around the edges of the cake and invert it onto a wire rack to cool completely.

Slice and serve warm or at room temperature. Enjoy your delicious apple cake!