Preheat the oven: Preheat the oven to 350°F / 180°C (160°C fan-assisted). Grease a 23 cm round springform pan with butter and line the bottom with baking parchment.

Prepare the apples: Peel, core, and chop 3 of the apples into small pieces. Slice the remaining apple thinly and set both aside.

Mix wet ingredients: In a large bowl, cream together the butter, sugar, and lemon zest using an electric mixer. Beat until light and fluffy, about 3-5 minutes.

Add milk and eggs: Gradually add the milk and eggs (one at a time), mixing well after each addition until smooth.

Incorporate dry ingredients: In a separate bowl, sift together the flour, baking powder, and salt. Slowly add the dry ingredients to the wet mixture, mixing just until combined.

Fold in the fruit and nuts: Gently fold in the chopped apples, pine nuts, and sultanas. Be careful not to overmix. Pour the batter into the prepared cake pan and arrange the thinly sliced apple on top.

Bake the cake: Place the cake on the middle rack of the preheated oven, and position a baking sheet on the rack below to catch any drips as the butter melts and caramelises. Bake for about 55 minutes, or until a toothpick inserted into the center comes out clean.

Cool and serve: Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely. Dust with icing sugar and serve with vanilla ice cream, if desired.