Heat the oven to 175°C conventional/350°F, or 160°C fan. Grease a 23-cm/9-inch round springform or deep-sided cake pan and line the base with baking paper.

Peel, core, and slice the apples into even wedges approximately 5 mm thick.

Melt the butter in a saucepan over medium heat. Add the brown sugar, golden syrup, cinnamon, and salt, then stir until smooth and gently bubbling for 1–2 minutes.

Pour the warm caramel into the prepared pan and arrange the apple slices in overlapping circles over the surface.

Cream the softened butter and caster sugar for 2–3 minutes, until pale and fluffy. Beat in the eggs one at a time, followed by the vanilla.

Whisk together the flour, baking powder, cinnamon, and salt. Add the dry ingredients in two or three additions, alternating with the milk, and mix only until combined.

Spoon the batter over the apples and spread gently to the edges without disturbing the arrangement.

Bake for 45–55 minutes, until golden and a skewer inserted into the cake comes out clean or with a few moist crumbs.

Let the cake stand in the pan for 10 minutes. Run a knife around the edge, place a serving plate over the pan, and invert carefully while the caramel is still warm. Remove the pan and baking paper.