Preheat the oven to 180°C (350°F) and grease a 9-inch round cake tin.

In a large mixing bowl, cream together the softened butter and caster sugar until light and fluffy.

Add the eggs one at a time, beating well after each addition.

Sift together the self-raising flour, baking powder, and ground cinnamon, then fold into the mixture gently.

Stir in the chopped apples, sultanas, and walnuts (if using).

Add the milk to loosen the mixture slightly, then mix until combined.

Pour the batter into the prepared cake tin and smooth the top with a spatula.

Bake in the preheated oven for 45-50 minutes, or until a skewer inserted into the center comes out clean.

Allow the cake to cool in the tin for 10 minutes before transferring it to a wire rack to cool completely.

Once cooled, dust with icing sugar before serving.