Step 1. Preheat the Oven and Prepare the Cake Pan

Preheat your oven to 180°C (350°F). Grease a 9-inch round cake pan and line it with parchment paper to prevent sticking.

Step 2. Mix the Dry Ingredients

In a mixing bowl, sift together the self-raising flour, baking powder, and ground cinnamon (if using).

Step 3. Cream Butter and Sugar

In a separate bowl, beat the softened butter and caster sugar together until light and fluffy, about 2-3 minutes with an electric mixer or whisk.

Step 4. Add Eggs and Vanilla

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until fully incorporated.

Step 5. Combine the Wet and Dry Ingredients

Gently fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix, as this can make the cake dense.

Step 6. Fold in the Apples

Fold the diced apples into the batter until they are evenly distributed.

Step 7. Pour the Batter and Add the Topping

Pour the batter into the prepared cake pan, spreading it evenly with a spatula. If desired, sprinkle the Demerara sugar over the top for a crunchy, caramelized finish.

Step 8. Bake the Cake

Bake in the preheated oven for 40-45 minutes, or until a skewer inserted into the center comes out clean and the top is golden brown.

Step 9. Cool and Serve

Allow the cake to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely. Serve with a dusting of powdered sugar, whipped cream, or custard, if desired.